nv50/ir: Add support for MAD 4-byte opcode

nv50/ir: Add support for MAD 4-byte opcode

Add emission rules for negative and saturate flags for MAD 4-byte opcodes,
and get rid of some of the constraints. Obviously tested with a wide variety
of shaders.

V2: Document MAD as supported short form
V3: Split up IMM from short-form modifiers

Signed-off-by: Roy Spliet <rspliet@eclipso.eu>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>