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

[Nikita Popov via llvm-commits]

nikic 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 [fc90685](https://github.com/llvm/llvm-project/commit/fc90685354ca36c935ebb0f315bb18592598a83b) (that's where I got this from originally)?

I don't think so. At that point the entry block is empty, so begin and terminator are the same. Possibly I picked terminator because that would preserve the old order of the alloca instructions.

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