[llvm] [LV] Use frozen start value for FindLastIV if needed. (PR #132691)

[Mel Chen via llvm-commits]

https://github.com/Mel-Chen approved this pull request.

LGTM after all current comments are fixed.
BTW, I can help update the llvm-testsuite to cover this case if there isn't already a unit test for it.

https://github.com/llvm/llvm-project/pull/132691