[42/46] Add vec_info::replace_stmt

[42/46] Add vec_info::replace_stmt

This patch adds a helper for replacing a stmt_vec_info's statement with
a new statement.

2018-07-31  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vectorizer.h (vec_info::replace_stmt): Declare.
	* tree-vectorizer.c (vec_info::replace_stmt): New function.
	* tree-vect-slp.c (vect_remove_slp_scalar_calls): Use it.
	* tree-vect-stmts.c (vectorizable_call): Likewise.
	(vectorizable_simd_clone_call): Likewise.

From-SVN: r263157