i386: Implement mmx_pblendv to optimize SSE conditional moves [PR98218]

i386: Implement mmx_pblendv to optimize SSE conditional moves [PR98218]

Implement mmx_pblendv to optimize V8HI, V4HI and V2SI mode
conditional moves for SSE4.1 targets.

2021-05-07  Uroš Bizjak  <ubizjak@gmail.com>

gcc/
	PR target/98218
	* config/i386/i386-expand.c (ix86_expand_sse_movcc):
	Handle V8QI, V4HI and V2SI modes.
	* config/i386/mmx.md (mmx_pblendvb): New insn pattern.
	* config/i386/sse.md (unspec): Move UNSPEC_BLENDV ...
	* config/i386/i386.md (unspec): ... here.