Identify bitmasks

[Rangi42]

Most of the bit/res/set operations now use constants instead of magic numbers, but some related operations remain. Look over the and/or/xor instructions and see which ones can involve constants: https://pastebin.com/rb6eJu7g (this list omits and $0f, and $f0, and xor $ff since nybbles and negation are understood).