[LIR] Re-instate r244880, reverted in r244884, factoring the handling of

[LIR] Re-instate r244880, reverted in r244884, factoring the handling of
AliasAnalysis in LoopIdiomRecognize.

The previous commit to LIR, r244879, exposed some scary bug in the loop
pass pipeline with an assert failure that showed up on several bots.
This patch got reverted as part of getting that revision reverted, but
they're actually independent and unrelated. This patch has no functional
change and should be completely safe. It is also useful for my current
work on the AA infrastructure.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244993 91177308-0d34-0410-b5e6-96231b3b80d8