[llvm] [JumpThreading] Hoist constant-size allocas and lifetime.start to the nearest common dominator (PR #205248)

[Nikita Popov via llvm-commits]

nikic wrote:

> Is it reasonable to hoist it to entry block in JumpThreading or the optimization is not worthy?

I don't expect this to be particularly useful in practice, because allocas should nearly always be in the entry block. If an alloca is ever not in the entry block, it's best to adjust the code producing it instead. (Sometimes this happens accidentally when doing block splitting on the entry block.)

https://github.com/llvm/llvm-project/pull/205248