Set the call nothrow flag more often

This patch sets the nothrow flag for various calls to internal functions
that are not inherently NOTHROW (and so can't be declared that way in
internal-fn.def) but that are used in contexts that can guarantee
NOTHROWness.

2017-08-29  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* gimplify.c (gimplify_call_expr): Copy the nothrow flag to
	calls to internal functions.
	(gimplify_modify_expr): Likewise.
	* tree-call-cdce.c (use_internal_fn): Likewise.
	* tree-ssa-math-opts.c (pass_cse_reciprocals::execute): Likewise.
	(convert_to_divmod): Set the nothrow flag.
	* tree-if-conv.c (predicate_mem_writes):  Likewise.
	* tree-vect-stmts.c (vectorizable_mask_load_store): Likewise.
	(vectorizable_call): Likewise.
	(vectorizable_store): Likewise.
	(vectorizable_load): Likewise.
	* tree-vect-patterns.c (vect_recog_pow_pattern): Likewise.
	(vect_recog_mask_conversion_pattern): Likewise.

From-SVN: r251401