[llvm-branch-commits] [MacroFusion] Add SingleFusion that accepts a single instruction pair (PR #85750)

[Wang Pengcheng via llvm-branch-commits]

https://github.com/wangpc-pp created https://github.com/llvm/llvm-project/pull/85750

We add a common class `SingleFusion` that accepts a single instruction
pair to simplify fusion definitions.