allow const_range allocation; preprove if-then-else (#2419)

python/tvm/hybrid/parser.py

@@ -339,6 +339,9 @@ class HybridParser(ast.NodeVisitor):
             else_body = visit_list_to_block(self.visit, node.orelse)
         else:
             else_body = util.make_nop()
+        # Return no IfThenElse if proven
+        if isinstance(cond, _expr.UIntImm):
+            return if_body if cond.value else else_body
         return _make.IfThenElse(cond, if_body, else_body)
 
 
@@ -429,7 +432,7 @@ class HybridParser(ast.NodeVisitor):
             for i in range(low, low + ext):
                 self.symbols[_name] = Symbol.ConstLoopVar, i
                 bodies.append(visit_list_to_block(self.visit, node.body))
-            return pack_list_to_block(bodies)
+            _body = pack_list_to_block(bodies)
 
         elif iter_var is None:
             _internal_assert(for_type is not None, "The loop bind function parse error!")
@@ -449,6 +452,9 @@ class HybridParser(ast.NodeVisitor):
             res = _make.AttrStmt(iter_var, 'thread_extent', ext, _body)
         elif not isinstance(for_type, tuple):
             res = _make.For(iter_var, _api.const(0, 'int32'), ext, for_type, 0, _body)
+        else:
+            res = _body
+
         self.symbols.pop(_name)
         return res