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

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> Btw, I'd probably make the Instruction* ctor of FMFSource explicit, so one has to write FMFSource(I). 

The implicit ctor is designed to keep compatibility with downstream projects.


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