nv50/ir: Fold IMM into MAD

nv50/ir: Fold IMM into MAD

Add a specific optimisation pass for NV50 to check whether SRC0 or SRC1 is
a MOV dst, IMM. If so: fold the IMM in and try to drop the MOV. Must be
done post-RA because it requires that SDST == SSRC2.

V2: improve readability and add comments to clarify decisions
V3: Remove redundant code... compiler already attempts to put the IMM in
SSRC1

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