Revert "InstCombine: Fold more shuffles of shuffles."

This reverts commit r180802

There's ongoing discussion about whether this is the right place to make
this transformation. Reverting for now while we figure it out.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180834 91177308-0d34-0410-b5e6-96231b3b80d8