Add a new loop-instsimplify pass, with the intention of replacing the instance

Add a new loop-instsimplify pass, with the intention of replacing the instance
of instcombine that is currently in the middle of the loop pass pipeline. This
commit only checks in the pass; it will hopefully be enabled by default later.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122719 91177308-0d34-0410-b5e6-96231b3b80d8