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

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

@mgudim I found that `isFIDef` is never true so nothing ever gets added to the map.

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