[llvm] [ValueTracking] Compute knownbits from known fp classes (PR #86409)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> Reduced test case:
> 
> ```
> define i32 @test(double %x) {
>   %cmp = fcmp oeq double %x, 0xFFF0000000000000
>   br i1 %cmp, label %if.then, label %if.else
> 
> if.then:
>   %cast = bitcast double %x to i64
>   %trunc = trunc i64 %cast to i32
>   ret i32 %trunc
> 
> if.else:
>   ret i32 0
> }
> ```
> 
> I will post a fix.

Thanks! I pushed the revert already in https://github.com/llvm/llvm-project/commit/2e165a2c4b2a0e9a9d34a721d756f9006d1502df though, so I guess it mostly makes sense as a new PR for relanding this change.

https://github.com/llvm/llvm-project/pull/86409