[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 #116073 and it doesn't seem to affect compile-time, which would have been my main concern.
> 
> @nikic Is it okay to just keep these negative tests and make this patch NFC? Positive tests have been added in [2ca25ab](https://github.com/llvm/llvm-project/commit/2ca25ab11d01ceacf359643b09aed7d53d0ff8dc).

Sure

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