[rs6000] Add _mm_blend_epi16 and _mm_blendv_epi8

[rs6000] Add _mm_blend_epi16 and _mm_blendv_epi8

Add compatibility implementations of _mm_blend_epi16 and _mm_blendv_epi8
intrinsics.

Respective test cases are copied almost verbatim (minor changes to
the dejagnu head lines) from i386.

2019-07-22  Paul A. Clarke  <pc@us.ibm.com>

[gcc]

	* config/rs6000/smmintrin.h (_mm_blend_epi16): New.
	(_mm_blendv_epi8): New.

[gcc/testsuite]

	* gcc.target/powerpc/sse4_1-check.h: New.
	* gcc.target/powerpc/sse4_1-pblendvb.c: New.
	* gcc.target/powerpc/sse4_1-pblendw.c: New.
	* gcc.target/powerpc/sse4_1-pblendw-2.c: New.

From-SVN: r273698