forwprop: Factor out memchr optimization to its own function

forwprop: Factor out memchr optimization to its own function

As more optimizations are added to forwprop's simplify_builtin_call,
this function is becoming harder and harder to understand. To help
simplify things, this factors out the memchr optimization to its own
function like what was done when memcmp optimization was added.

Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* tree-ssa-forwprop.cc (simplify_builtin_call): Factor out the memchr
	optimization to ...
	(simplify_builtin_memchr): Here. New function.

Signed-off-by: Andrew Pinski <andrew.pinski@oss.qualcomm.com>