[llvm] [ReachingDefAnalysis] Fix management of MBBFrameObjsReachingDefs (PR #124943)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

I had some trouble writing a test since we were sort of getting lucky. What I mean by that is `lookup` was returning a empty list of defs as a default value which was exactly what would happen when there is no reaching defs. I think that's why it did not impact the tests negatively. But this is definitely more robust. That makes that part of the patch "NFC-ish" i think, although I don't want to call it NFC because it may be fixing a tricky to find bug.

WRT to the Frame2InstrIdx bug, I would be surprised if that was NFC. I will try and write a test for it.

https://github.com/llvm/llvm-project/pull/124943