[x86] Teach the instruction printer to decode immediate operands to
BLENDPS, BLENDPD, and PBLENDW instructions into pretty shuffle comments.

These will be used in my next commit as part of test cases for AVX
shuffles which can directly use blend in more places.
[x86] Add the initial skeleton of type-based dispatch for AVX vectors in
the new shuffle lowering and an implementation for v4 shuffles.

This allows us to handle non-half-crossing shuffles directly for v4
shuffles, both integer and floating point. This currently misses places
where we could perform the blend via UNPCK instructions, but otherwise
generates equally good or better code for the test cases included to the
existing vector shuffle lowering. There are a few cases that are
entertainingly better. ;]