[MacroFusion] Expose useful static methods. NFC.

hasLessThanNumFused and fuseInstructionPair are useful for
DAG mutations similar to MacroFusion, but which cannot use
MacroFusion as a whole (such as fusing non-dependent instruction).

Reviewed By: MatzeB

Differential Revision: https://reviews.llvm.org/D111070

Change-Id: I3a5d56aba0471d45ef64cebb9b724030e2eae2f3