[LIR] Start leveraging the fundamental guarantees of a loop in

[LIR] Start leveraging the fundamental guarantees of a loop in
simplified form to remove redundant checks and simplify the code for
popcount recognition. We don't actually need to handle all of these
cases.

I've left a FIXME for one in particular until I finish inspecting to
make sure we don't actually *rely* on the predicate in any way.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244879 91177308-0d34-0410-b5e6-96231b3b80d8