[llvm] [ConstantFPRange] Implement `ConstantFPRange::makeAllowedFCmpRegion` (PR #110082)

[via llvm-commits]

vporpo wrote:

I just noticed that `ConstantFPRangeTest.makeAllowedFCmpRegion` takes a very long time to run, like 8 minutes, and slows down check-llvm. PTAL.

https://github.com/llvm/llvm-project/pull/110082