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.

gcc/testsuite/ChangeLog

+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.
+
 2020-01-31  Richard Sandiford  <richard.sandiford@arm.com>
 
 	* lib/target-supports.exp (check_effective_target_aarch64_asm_bf16_ok):

gcc/testsuite/g++.dg/opt/pr91838.C

+/* { dg-do compile { target c++11 } } */
+/* { dg-additional-options "-O2 -Wno-psabi -w" } */
+/* { dg-additional-options "-masm=att" { target i?86-*-* x86_64-*-* } } */
+
+using T = unsigned char; // or ushort
+using V [[gnu::vector_size(8)]] = T;
+V f(V x) {
+  return x >> 8 * sizeof(T);
+}
+
+/* { dg-final { scan-assembler {pxor\s+%xmm0,\s+%xmm0} { target { { i?86-*-* x86_64-*-* } && lp64 } } } } */

gcc/testsuite/g++.dg/pr91838.C

-/* { dg-do compile } */
-/* { dg-additional-options "-O2" } */
-/* { dg-skip-if "" { *-*-* } {-std=c++98} } */
-
-using T = unsigned char; // or ushort, or uint
-using V [[gnu::vector_size(8)]] = T;
-V f(V x) {
-  return x >> 8 * sizeof(T);
-}
-
-/* { dg-final { scan-assembler {pxor\s+%xmm0,\s+%xmm0} { target x86_64-*-* } } } */