[llvm] [InstCombine] Fold "extract (select (cond, insert(agg, elem), FV))" (PR #115969)

[Nikita Popov via llvm-commits]

nikic wrote:

> We may generalize to use simplifyInstruction for other instructions in the future.

I'd rather directly do that than add a special case for extractelement. It's easier to generalize this than not. I did a quick test at https://github.com/llvm/llvm-project/pull/116073 and it doesn't seem to affect compile-time, which would have been my main concern.

https://github.com/llvm/llvm-project/pull/115969