[llvm] Let M68kMCCodeEmitter set Scratch size. (PR #69898)

[Min-Yih Hsu via llvm-commits]

https://github.com/mshockwave commented:

This is a pretty neat optimization as the original version might allocated more bits than we needed for `Scratch`. Thank you!
I only have a few comments.

https://github.com/llvm/llvm-project/pull/69898