TIR is the new namespace for low-level IR
for tensor-level optimizations and loop transformations.

This PR establishes the namespace and files.

- lowered_func.h,buffer.h,data_layout.h -> tir/buffer.h,tir/data_layout.h,tir/lowered_func.h
- ir.h -> tir/expr.h, tir/stmt.h
- ir_functor_ext.h -> tir/expr_functor.h, tir/stmt_functor.h

Move the conversion extensions to the specific class definitions
so that we longer need to include packed_func_ext.

* [arith] add ShapeVar representing non-neg valued variable in a tensor shape

* bounder remover; deal with div in int_set differently

* fix bounder_remover

* migrate unittest to use shape_var

* use tvm.shape_var in integration & relay tests

* add test case; fix Var register

* fix lint

* fix lint again

* add default ShapeVar visitor in Relay

* fix override

* fix ShapeVar visit bug

* revert IntervalSet for shape_var

* remove bound_remover

* remove is_var; use constructor for shapevar/var instead

* ShapeVar -> SizeVar; add constructor comments

* shape_var -> size_var in doc

* tindex -> size

* [REFACTOR][IR] Unify IntImm and UIntImm

This PR unifies UIntImm and IntImm to simplify the codebase.
Unsigned integer constants will also be stored as IntImm.

For uint constant that does not fit into int64(rare case), we introduced
an intrinsic tvm_big_uint_imm to construct such intgers by its
lower and higher 32bits.

* [REFACTOR][IR] Remove UIntImm to use IntImm

* rename big->large

* [REFACTOR][IR] tvm::Expr -> PrimExpr(Primitive Expr)

As part of unified IR, we will need to unify relay::Expr
and the current tvm::Expr under the same base type.

From the techinical point of view. tvm::Expr is a "primitive"
expression that only contains POD types and handles and does
not do life-cycle management.

This PR renames Expr->PrimExpr to clarify that.
We will send a subsequent PR to introduce the base expr class.

* Remove legacy VarExpr and ExprHash/Equal

* [REFACTOR][IR] Variable -> VarNode

* [REFACTOR][IR] Add/Sub/Mul/Div -> AddNode/SubNode etc.

* [REFACTOR][IR] Min/Max/FloorDiv/FloorMod -> MinNode/MaxNode etc.

* [REFACTOR][IR] EQ/NE/LT/LE/GT/GE/Select -> EQNode/NENode etc.

* [REFACTOR][IR] Add Node suffix to Select/Call/Load/Ramp/Shuffle/Let

* [REFACTOR][IR] Add node suffix to IntImm/UIntImm/FloatImm/StringImm

* [REFACTOR][IR] Add Node suffix to Any, AttrStmt, AssertStmt

* [REFACTOR][IR] Add Node suffix to Store/Provide/Allocate/Free

* [REFACTOR][IR] Add Node suffix to ProducerConsumer

* Fix lint

* style updates, test fixes

* [REFACTOR][IR] Introduce SeqStmt to replace Block

ir::Block was used to represent a sequence of Stmts in the original low-level IR.
The nested ir::Block structure is not really friendly for recursive visits,
especially when the statements are unrolled.

This PR introduce a SeqStmt that directly stores a sequence of statements in an Array container.
The new SeqStmt will be used as a replacement of the original Block structure.

* [REFACTOR] Migrate use of Block to SeqStmt.

* [REFACTOR] Remove Block

* Add more comments per yizhi's comment

* CombineContextCall

* Migrate BoundChecker

* Migrate CoprocSync

* Migrate detect_device

* Migrate loop_partition

* Migrate infer_fragement

* Migrate inject_copy_intrin

* Migrate inject double buffer

* Migrate lower_intrin and simplify

* Migrate storage flatten

* Migrate inject prefetch

* Migrate inject_virtual_thread

* migrate inline

* Migrate lift attr scope

* Migrate custom datatypes

* migrate lower_thread_all_reduce

* Migrate lower_tvm_builtin

* migrate lower_warp memory

* Migrate make_api.cc

* Migrate remap_thread_axis

* Migrate remove_no_op

* migrate rewrite_unsafe_select

* Migrate skip_assert simple_passes

* Migrate split_host_device

* Migrate ssa

* Migrate storage_access

* Migrate storage_rewrite

* Migrate tensor_core

* Migrate unroll_loop

* Migrate vectorize

* Migrate verify compact_buffer gpu_code

* Migrate verify_memory

* Migrate storage_sync

* Remove unused refs to mutator

* Migrate hybrid_op

* Migrate tensorize

* Migrate schedule ops

* Migrate schedule_dataflow_rewrite

* Migrate auto_inline_elemwise

* Remove unecessary ref to visitor

* remove unecessary ref

* Migrate bound_deducer

* Migrate domain_touched

* Migrate autotvm feature touch extractor

* Add annotations

IRMutator and IRVisitor were the main data structures for doing low level IR visiting.
As the project evolves, we start to introduce more powerful variants such as StmtFunctor and ExprFunctor.
This PR brings new classes that allows us to migrate the visitor mutator to be sub-class of these functors.

List of changes:

- Create separate class for ExprMutator and StmtMutator, following convention used in relay.
- Introduce copy-on-write to StmtMutator that can later benefit the statement mutations
  if we use move semantics and keep a single copy of stmt.
- Move two generic visit mutate util to use the new classes.

We will send followup PRs to migrate the existing passes that use the legacy visitors
to the new one.