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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> 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.

Good point. I updated the patch to do that. Was there a reason that you opted to insert before the terminator instruction in fc90685354ca36c935ebb0f315bb18592598a83b (that's where I got this from originally)?

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