Revert r367891 - "[InstCombine] combine mul+shl separated by zext"

This reverts commit 5dbb90bf.

As noted in the post-commit thread for r367891, this can create
a multiply that is lowered to a libcall that may not exist.

We need to improve the backend decomposition for integer multiply
before trying to re-land this (if it's still worthwhile after
doing the backend work).

llvm-svn: 369174