[llvm] [INSTR COMBINE] implement combine select into cast (PR #114612)

[Nikita Popov via llvm-commits]

nikic wrote:

> > But even before that, wasn't the consensus from #107034 that this is better handled in the backend on the SDAG level, rather than in InstCombine?
> 
> That's right, they noted it in the ticket. Maybe the comments here can help finding a better place for it? https://github.com/llvm/llvm-project/pull/107732/files#r1749823010
> 
> FYI @Cr0a3 was asking for a good first issue and we spotted this one quickly. After all, it has that label. Not sure it's still the case if it must be implemented in ISel? Anyway, good first attempt :)

Yeah, looking at the discussion in that PR, this issue may not be as simple as it appeared on the surface :)

If you're looking for a simple middle-end issue, I just opened https://github.com/llvm/llvm-project/issues/114820, though maybe it's not a particularly exciting one...

https://github.com/llvm/llvm-project/pull/114612