mesa: document instructions ir_to_mesa emits

mesa: document instructions ir_to_mesa emits

GLSL stopped using:
   BRA, EXP, LOG, LRP, NRM3, NRM4, XPD.

GLSL started using:
   KIL, SCS, SSG, SWZ.
(omg why SWZ? isn't proc_src_register flexible enough?)

GLSL doesn't use these opcodes some Radeons do support:
   ARR, DP2A, DST, LRP, XPD.

These opcodes are now unused:
   AND, NOT, NRM3, NRM4, OR, XOR.
(plus maybe the NV extensions which are unused by Gallium)

In addition to that, we don't use two-dimensional indirect addressing,
which the Mesa IR can do.