vc4: Switch the unpack ops to being unpack flags on a mov.

vc4: Switch the unpack ops to being unpack flags on a mov.

This paves the way for copy propagating our unpacks.  We end up with a
small change on shader-db:

total instructions in shared programs: 89390 -> 89251 (-0.16%)
instructions in affected programs:     19041 -> 18902 (-0.73%)

which appears to be because we no longer convert MOVs for an FMAX dst,
r4.unpack, r4.unpack (instead of the previous MOV dst, r4.unpack), and
this ends up with a slightly better schedule.