This finishes the optimizations mentioned in #6188 (closed) by adding the MOV+MAD case and few more small tweaks.
The other possiblities (MUL+ADD and ADD+ADD) turns out to not be worth it. I have a working patch, however there is just few saved instructions in the whole shader-db because it works agains the "source conflict resolve" pass so it might be worth revisiting after !14499 lands
total shader-db RV530 stats:
total instructions in shared programs: 136382 -> 135151 (-0.90%) instructions in affected programs: 52418 -> 51187 (-2.35%) total temps in shared programs: 18939 -> 18926 (-0.07%) temps in affected programs: 641 -> 628 (-2.03%)