[M68k] Fix the crash of fast register allocator

`MOVEM` is used to spill the register, which will cause problem with 1 byte data, since it only supports word (2 bytes) and long (4 bytes) size.

We change to use the normal `move` instruction to spill 1 byte data.

Fixes #57660

Reviewed By: myhsu

Differential Revision: https://reviews.llvm.org/D133636