ldist: Recognize strlen and rawmemchr like loops

ldist: Recognize strlen and rawmemchr like loops

This patch adds support for recognizing loops which mimic the behaviour
of functions strlen and rawmemchr, and replaces those with internal
function calls in case a target provides them.  In contrast to the
standard strlen and rawmemchr functions, this patch also supports
different instances where the memory pointed to is interpreted as 8, 16,
and 32-bit sized, respectively.

gcc/ChangeLog:

	* builtins.c (get_memory_rtx): Change to external linkage.
	* builtins.h (get_memory_rtx): Add function prototype.
	* doc/md.texi (rawmemchr<mode>): Document.
	* internal-fn.c (expand_RAWMEMCHR): Define.
	* internal-fn.def (RAWMEMCHR): Add.
	* optabs.def (rawmemchr_optab): Add.
	* tree-loop-distribution.c (find_single_drs): Change return code
	behaviour by also returning true if no single store was found
	but a single load.
	(loop_distribution::classify_partition): Respect the new return
	code behaviour of function find_single_drs.
	(loop_distribution::execute): Call new function
	transform_reduction_loop in order to replace rawmemchr or strlen
	like loops by calls into builtins.
	(generate_reduction_builtin_1): New function.
	(generate_rawmemchr_builtin): New function.
	(generate_strlen_builtin_1): New function.
	(generate_strlen_builtin): New function.
	(generate_strlen_builtin_using_rawmemchr): New function.
	(reduction_var_overflows_first): New function.
	(determine_reduction_stmt_1): New function.
	(determine_reduction_stmt): New function.
	(loop_distribution::transform_reduction_loop): New function.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/ldist-rawmemchr-1.c: New test.
	* gcc.dg/tree-ssa/ldist-rawmemchr-2.c: New test.
	* gcc.dg/tree-ssa/ldist-strlen-1.c: New test.
	* gcc.dg/tree-ssa/ldist-strlen-2.c: New test.
	* gcc.dg/tree-ssa/ldist-strlen-3.c: New test.