[X86][MMX] Reapply: Add MMX instructions to foldable tables

Reapply r230248.

Teach the peephole optimizer to work with MMX instructions by adding
entries into the foldable tables. This covers folding opportunities not
handled during isel.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230499 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed