r300: possibility of merging ADD and MUL into MAD when effectively only 2 sources are used

Consider following snippet of code:

MUL temp2.xy temp0.xy temp1.xy
ADD temp2.z temp0.z temp0.1

Because only 2 sources are used we aren't failing into case of MAD with macro.

(The same with MUL + MOV or ADD + MOV)

Tagging as always @ondracka

EDIT by @ondracka:

Just adding a list of all the other options we can do together with what is done already (merged in #17560):

• MOV + MOV
• MOV + ADD
• MOV + MUL
• MOV + MAD (if they share a source or at least one source is None) WIP branch: https://gitlab.freedesktop.org/ondracka/mesa/-/commits/r300_merge_channels2
• ADD + MUL (if they share a source or at least one source is None)
• ADD + ADD (if they share a source or at least one source is None)