vc4: Move discard handling to the condition flag.

vc4: Move discard handling to the condition flag.

Now that the field exists in the instruction, we can make discards less
special.  As a bonus, that means that we should be able to merge some more
.sf instructions together when we get around to that.

This causes some scheduling changes, as it allows tlb_color_reads to be
delayed past the discard condition setup.  Since the tlb_color_read ends
up later, this may mean performance improvements, but I haven't tested.

total instructions in shared programs: 78114 -> 78035 (-0.10%)
instructions in affected programs:     1922 -> 1843 (-4.11%)
total estimated cycles in shared programs: 234318 -> 234329 (0.00%)
estimated cycles in affected programs:     8200 -> 8211 (0.13%)