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

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

Here is the memory usage compared to before your patch went in:

https://llvm-compile-time-tracker.com/compare.php?from=8baa0d9d545f9daf0d82596cb90f35456efb1153&to=4ef33af3bb780b9f7ef13dea8c15479dfd30f28f&stat=max-rss

@nikic is this reasonable?

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