vc4: Add support for coalescing ALU ops into tex_[srtb] MOVs.

vc4: Add support for coalescing ALU ops into tex_[srtb] MOVs.

This isn't as complete as I would like (can't merge interpolation because
of the implicit r5 dependency, doesn't work with control flow), but this
was cheap and easy.

Improves 3DMMES Taiji performance by 1.15353% +/- 0.299896% (n=29, 16)

total instructions in shared programs: 99810 -> 99059 (-0.75%)
instructions in affected programs:     10705 -> 9954 (-7.02%)