It forms memset and memcpy's, and will someday form popcount and
other stuff. All of this is bad when compiling the implementation
of memset, memcpy, popcount, etc.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122854 91177308-0d34-0410-b5e6-96231b3b80d8