[x86] Teach the new shuffle lowering's blend functionality to use AVX2's
VPBLENDD where appropriate even on 128-bit vectors.

According to Agner's tables, this instruction is significantly higher
throughput (can execute on any port) on Haswell chips so we should
aggressively try to form it when available.

Sadly, this loses our delightful shuffle comments. I'll add those back
for VPBLENDD next.