-finline-stringops: don't assume ptr_mode ptr in memset [PR112804]

On aarch64 -milp32, and presumably on other such targets, ptr can be
in a different mode than ptr_mode in the testcase.  Cope with it.


for  gcc/ChangeLog

	PR target/112804
	* builtins.cc (try_store_by_multiple_pieces): Use ptr's mode
	for the increment.

for  gcc/testsuite/ChangeLog

	PR target/112804
	* gcc.target/aarch64/inline-mem-set-pr112804.c: New.