[llvm] RegAllocGreedy: Fix use after free during last chance recoloring (PR #120697)

[Vladimir Vereschaka via llvm-commits]

vvereschaka wrote:

also, got failed test on `llvm-clang-x86_64-expensive-checks-ubuntu` builder
https://lab.llvm.org/buildbot/#/builders/187/builds/3566

https://github.com/llvm/llvm-project/pull/120697