[M68k][NFC] Rename MximmSExt8/16/32 to Mxi8/16/32immSExt8/16/32

The MximmSExt8/16/32 should be "any immediate that can be represented by
8/16/32-bit signed integer", hence it shouldn't express an explicit
type. Rename those into Mxi8/16/32immSExt8/16/32.

NFC.