[llvm] AMDGPU: Fix handling of -0 in round lowering (PR #65761)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > The GlobalISel output [...] fails to form BFI.
> 
> That's not visible in any of the codegen tests is it? We should have something that shows the final codegen for globalisel.
> 
> Code change looks good.

Yes, this one is inconsistently tested and only has the mir legalizer test

https://github.com/llvm/llvm-project/pull/65761