testsuite: Fix up pr91838.C test [PR91838]

testsuite: Fix up pr91838.C test [PR91838]

The test FAILs on i686-linux with:
FAIL: g++.dg/pr91838.C   (test for excess errors)
Excess errors:
/home/jakub/src/gcc/gcc/testsuite/g++.dg/pr91838.C:7:8: warning: MMX vector return without MMX enabled changes the ABI [-Wpsabi]
/home/jakub/src/gcc/gcc/testsuite/g++.dg/pr91838.C:7:3: warning: MMX vector argument without MMX enabled changes the ABI [-Wpsabi]
and on x86_64-linux with -m32 testing with failure to match the
expected pattern in there (or both with e.g. -m32/-mno-mmx/-mno-sse testing).
The test is also in a wrong directory, has non-standard specification that
it requires c++11 or later.

2020-01-31  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/91838
	* g++.dg/pr91838.C: Moved to ...
	* g++.dg/opt/pr91838.C: ... here.  Require c++11 target instead of
	dg-skip-if for c++98.  Pass -Wno-psabi -w to avoid psabi style
	warnings on vector arg passing or return.  Add -masm=att on i?86/x86_64.
	Only check for pxor %xmm0, %xmm0 on lp64 i?86/x86_64.