[llvm] [IRBuilder] Add a helper function to intersect FMFs from two instructions (PR #122059)

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

LGTM

Btw, I'd probably make the Instruction* ctor of FMFSource explicit, so one has to write `FMFSource(I)`. I found some of the method calls a bit confusing to read, esp the ones that don't have FMF in the name, as it's not really clear that the argument is used to get FMF and isn't just another value argument.

https://github.com/llvm/llvm-project/pull/122059