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

[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. =]

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229309 91177308-0d34-0410-b5e6-96231b3b80d8