[MergeICmps] Re-commit rL324317 "Enable the MergeICmps Pass by default."

With fixes from rL324341.

Original commit message:

[MergeICmps] Enable the MergeICmps Pass by default.

Summary: Now that PR33325 is fixed, this should always improve the generated code.

Reviewers: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D42793

llvm-svn: 324465