This patch short-circuits the builtins.c expansion code for a particular
gimple call if:

- the function has an associated internal function
- the target implements that internal function
- the call has no side effects

This allows a later patch to remove the builtins.c code, once calls with
side effects have been handled.

Tested on x86_64-linux-gnu, aarch64-linux-gnu and arm-linux-gnueabi.

gcc/
	* builtins.h (called_as_built_in): Declare.
	* builtins.c (called_as_built_in): Make external.
	* internal-fn.h (expand_internal_call): Define a variant that
	specifies the internal function explicitly.
	* internal-fn.c (expand_load_lanes_optab_fn)
	(expand_store_lanes_optab_fn, expand_ANNOTATE, expand_GOMP_SIMD_LANE)
	(expand_GOMP_SIMD_VF, expand_GOMP_SIMD_LAST_LANE)
	(expand_GOMP_SIMD_ORDERED_START, expand_GOMP_SIMD_ORDERED_END)
	(expand_UBSAN_NULL, expand_UBSAN_BOUNDS, expand_UBSAN_VPTR)
	(expand_UBSAN_OBJECT_SIZE, expand_ASAN_CHECK, expand_TSAN_FUNC_EXIT)
	(expand_UBSAN_CHECK_ADD, expand_UBSAN_CHECK_SUB)
	(expand_UBSAN_CHECK_MUL, expand_ADD_OVERFLOW, expand_SUB_OVERFLOW)
	(expand_MUL_OVERFLOW, expand_LOOP_VECTORIZED)
	(expand_mask_load_optab_fn, expand_mask_store_optab_fn)
	(expand_ABNORMAL_DISPATCHER, expand_BUILTIN_EXPECT, expand_VA_ARG)
	(expand_UNIQUE, expand_GOACC_DIM_SIZE, expand_GOACC_DIM_POS)
	(expand_GOACC_LOOP, expand_GOACC_REDUCTION, expand_direct_optab_fn)
	(expand_unary_optab_fn, expand_binary_optab_fn): Add an internal_fn
	argument.
	(internal_fn_expanders): Update prototype.
	(expand_internal_call): Define a variant that specifies the
	internal function explicitly. Use it to implement the previous
	interface.
	* cfgexpand.c (expand_call_stmt): Try to expand calls to built-in
	functions as calls to internal functions.

From-SVN: r230487