Revert "[MergeICmps] Disable mergeicmps if the target does not want to handle memcmp expansion."

Breaks
clang-stage1-cmake-RA-incremental/llvm/test/Transforms/MergeICmps/X86/tuple-four-int8.ll

This reverts commit 3038c459d67f8898ffa295d54a013b280690abfa.

llvm-svn: 314972