rs6000: Support SSE4.1 "cvt" intrinsics

Function signatures and decorations match gcc/config/i386/smmintrin.h.

Also, copy tests for:
- _mm_cvtepi8_epi16, _mm_cvtepi8_epi32, _mm_cvtepi8_epi64
- _mm_cvtepi16_epi32, _mm_cvtepi16_epi64
- _mm_cvtepi32_epi64,
- _mm_cvtepu8_epi16, _mm_cvtepu8_epi32, _mm_cvtepu8_epi64
- _mm_cvtepu16_epi32, _mm_cvtepu16_epi64
- _mm_cvtepu32_epi64

from gcc/testsuite/gcc.target/i386.

sse4_1-pmovsxbd.c, sse4_1-pmovsxbq.c, and sse4_1-pmovsxbw.c were
modified from using "char" types to "signed char" types, because
the default is unsigned on powerpc.

2021-10-11  Paul A. Clarke  <pc@us.ibm.com>

gcc
	* config/rs6000/smmintrin.h (_mm_cvtepi8_epi16, _mm_cvtepi8_epi32,
	_mm_cvtepi8_epi64, _mm_cvtepi16_epi32, _mm_cvtepi16_epi64,
	_mm_cvtepi32_epi64, _mm_cvtepu8_epi16, _mm_cvtepu8_epi32,
	_mm_cvtepu8_epi64, _mm_cvtepu16_epi32, _mm_cvtepu16_epi64,
	_mm_cvtepu32_epi64): New.

gcc/testsuite
	* gcc.target/powerpc/sse4_1-pmovsxbd.c: Copy from gcc.target/i386,
	adjust dg directives to suit.
	* gcc.target/powerpc/sse4_1-pmovsxbq.c: Same.
	* gcc.target/powerpc/sse4_1-pmovsxbw.c: Same.
	* gcc.target/powerpc/sse4_1-pmovsxdq.c: Same.
	* gcc.target/powerpc/sse4_1-pmovsxwd.c: Same.
	* gcc.target/powerpc/sse4_1-pmovsxwq.c: Same.
	* gcc.target/powerpc/sse4_1-pmovzxbd.c: Same.
	* gcc.target/powerpc/sse4_1-pmovzxbq.c: Same.
	* gcc.target/powerpc/sse4_1-pmovzxbw.c: Same.
	* gcc.target/powerpc/sse4_1-pmovzxdq.c: Same.
	* gcc.target/powerpc/sse4_1-pmovzxwd.c: Same.
	* gcc.target/powerpc/sse4_1-pmovzxwq.c: Same.