Make Mips fast-isel track the form of AArch64 where practical.
This makes it easier for people to review the code, to borrow similar code, and to see how to eventually move a lot of this
target code for fast-isels into target independent code.
These are just cosmetic changes. Should be no functional difference.
Nit: I realize the AArch64 code uses 'O' as well but please pick something else. It looks too much like '0'.