[llvm] [InstCombine] Use the select condition to try to constant fold binops into select (PR #84696)

[via llvm-commits]

goldsteinn wrote:

> ```
> define i32 @before(i1 %cond) {
>   %sel = select i1 %cond, i32 32, i32 0
>   call void @use(i32 %sel)
>   %ret = or disjoint i32 %sel, 16
>   ret i32 %ret
> }
> 
> define i32 @after(i1 %cond) {
>   %sel = select i1 %cond, i32 32, i32 0
>   call void @use(i32 %sel)
>   %ret = select i1 %cond, i32 48, i32 16
>   ret i32 %ret
> }
> ```
> 
> I don't think the latter form is better than the former. Do we need the one-use check?

Yeah, Although I would would think this is somewhat instruction dependent.
I.e `div`/`mul`, I would probably prefer the constant folded select, `or`/`and`,
the logic op.

Ill update with a flag for if we can accept more than 1 use.

https://github.com/llvm/llvm-project/pull/84696