[llvm] [InstCombine] Handle isNanOrInf idioms (PR #80414)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> @dtcxzyw FYI, this patch and your similar recent changes introduce runtime library calls with fp128 (https://godbolt.org/z/PWcjK86vz) so it's a bit of a de-optimization in that case. I think @FreddyLeaf will look into this when he returns from holiday. We should be able to fix this in the selection DAG.
> 
> @pranavk has been working on fp128-related issues, so may also want to be aware of this.

I will reverse the transform in SDAG to fix the problem.
See also https://github.com/llvm/llvm-project/pull/76338#issuecomment-1929678129.

https://github.com/llvm/llvm-project/pull/80414