PR tree-optimization/83907: Improved memset handling in strlen pass.

PR tree-optimization/83907: Improved memset handling in strlen pass.

This patch implements the missed optimization enhancement PR 83907,
by handling memset with a constant byte value in tree-ssa's strlen
optimization pass.  Effectively, this treats memset(dst,'x',3) as
it would memcpy(dst,"xxx",3).

This patch also includes a tweak to handle_store to address another
missed optimization observed in the related test case pr83907-2.c.
The consecutive byte stores to memory get coalesced into a vector
write of a vector const, but unfortunately tree-ssa-strlen's
handle_store didn't previously handle the (unusual) case where the
stored "string" starts with a zero byte but also contains non-zero
bytes.

2022-05-13  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR tree-optimization/83907
	* tree-ssa-strlen.cc (handle_builtin_memset): Record a strinfo
	for memset with an constant char value.
	(handle_store): Improved handling of stores with a first byte
	of zero, but not storing_all_zeros_p.

gcc/testsuite/ChangeLog
	PR tree-optimization/83907
	* gcc.dg/tree-ssa/pr83907-1.c: New test case.
	* gcc.dg/tree-ssa/pr83907-2.c: New test case.