x86: Avoid -Wuninitialized warnings on _mm*_undefined_* in C++ [PR105593]

x86: Avoid -Wuninitialized warnings on _mm*_undefined_* in C++ [PR105593]

In https://gcc.gnu.org/pipermail/gcc-patches/2023-January/609844.html
I've posted a patch to allow ignoring -Winit-self using GCC diagnostic
pragmas, such that one can mark self-initialization as intentional
disabling of -Wuninitialized warnings.

The following incremental patch uses that in the x86 intrinsic
headers.

2023-01-16  Jakub Jelinek  <jakub@redhat.com>

	PR c++/105593
gcc/
	* config/i386/xmmintrin.h (_mm_undefined_ps): Temporarily
	disable -Winit-self using pragma GCC diagnostic ignored.
	* config/i386/emmintrin.h (_mm_undefined_pd, _mm_undefined_si128):
	Likewise.
	* config/i386/avxintrin.h (_mm256_undefined_pd, _mm256_undefined_ps,
	_mm256_undefined_si256): Likewise.
	* config/i386/avx512fintrin.h (_mm512_undefined_pd,
	_mm512_undefined_ps, _mm512_undefined_epi32): Likewise.
	* config/i386/avx512fp16intrin.h (_mm_undefined_ph,
	_mm256_undefined_ph, _mm512_undefined_ph): Likewise.
gcc/testsuite/
	* g++.target/i386/pr105593.C: New test.