[llvm] Revert "[StackColoring] Delete dead stack slots (#75351)" (PR #75655)

[Paul Kirth via llvm-commits]

ilovepi wrote:

when I initially made the revert something in flang broke, but I couldn't get it to reproduce locally.  I want to be sure I don't have to forward fix too many things, so I'll wait for the buildkite job to finish, which should be pretty fast.

https://github.com/llvm/llvm-project/pull/75655