update_mir_test_checks: Accept IR as input as well as MIR

We need to handle IR for tests that want to do lowering (or just
-stop-after with IR as input). I've run this on one AArch64 test to
demonstrate what it looks like.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@321048 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed