[x86] Teach the decomposed shuffle/blend lowering to use an early blend

when that will allow it to lower with a single permute instead of
multiple permutes.

It tries to detect when it will only have to do a single permute in
either case to maximize folding of loads and such.

This cuts a *lot* of the avx2 shuffle permute counts in half. =]

llvm-svn: 229309