[x86] Begin emitting PBLENDW instructions for integer blend operations
when SSE4.1 is available.

This removes a ton of domain crossing from blend code paths that were
ending up in the floating point code path.

This is just the tip of the iceberg though. The real switch is for
integer blend lowering to more actively rely on this instruction being
available so we don't hit shufps at all any longer. =] That will come in
a follow-up patch.

Another place where we need better support is for using PBLENDVB when
doing so avoids the need to have two complementary PSHUFB masks.