[llvm] Reapply "[ControlHeightReduction] Drop lifetime annotations where necessary" (PR #160640)

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

LGTM, though an alternative here would be to just change the insertion point from terminator to begin. We generally place the allocas at the start of the entry block, not its end.

https://github.com/llvm/llvm-project/pull/160640