Fold blendv builtins into gimple.

Fold __builtin_ia32_pblendvb128 (a, b, c) as VEC_COND_EXPR (c < 0, b,
a), similar for float version but with mask operand VIEW_CONVERT_EXPR
to same sized integer vectype.

gcc/ChangeLog:

	* config/i386/i386-builtin.def (IX86_BUILTIN_BLENDVPD256,
	IX86_BUILTIN_BLENDVPS256, IX86_BUILTIN_PBLENDVB256,
	IX86_BUILTIN_BLENDVPD, IX86_BUILTIN_BLENDVPS,
	IX86_BUILTIN_PBLENDVB128): Replace icode with
	CODE_FOR_nothing.
	* config/i386/i386.c (ix86_gimple_fold_builtin): Fold blendv
	builtins.
	* config/i386/sse.md (*<sse4_1_avx2>_pblendvb_lt_subreg_not):
	New pre_reload splitter.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/funcspec-8.c: Replace
	__builtin_ia32_blendvpd with  __builtin_ia32_roundps_az.
	* gcc.target/i386/blendv-1.c: New test.
	* gcc.target/i386/blendv-2.c: New test.