[llvm] [GlobalIsel] Combine logic of floating point compares (PR #81886)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

The motivation was to fold more compares. The DAG combiner has the same combine in `foldLogicOfSetCCs`, but it is tight to the ISD namespace. The Instcombine method of calculating the new predicate is more portable.

https://github.com/llvm/llvm-project/pull/81886