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/3kVig3fL (this list omits and $0f, and $f0, and xor $ff since nybbles and negation are understood).

[Rangi42]

Note that the very first ones on that list, the and $3f ; sound lengths, are related to the rNR11 and rNR21 registers, which I would borrow hardware.inc's constant for... if one existed. gbdev/hardware.inc#46

[Rangi42]

Note that audio_constants.asm lists some constants related to "wSoundInput", which does not exist; they seem mroe appropriate for rNR52.