[llvm] [TableGen] Enhance testability of TableGen-based macro fusion (PR #73075)

[Craig Topper via llvm-commits]

topperc wrote:

Could we have tblgen autogenerate the macrofusion bits onto the end of the existing subtarget feature bits and autogenerate `has*()` functions for them like other subtarget features. It seems like we're now duplicating some of the -mattr functionality.

https://github.com/llvm/llvm-project/pull/73075