vc4: Convert vc4_opt_dead_code to work in the presence of control flow.

vc4: Convert vc4_opt_dead_code to work in the presence of control flow.

With control flow, we can't be sure that we'll see the uses of a variable
before its def as we walk backwards.  Given that NIR is eliminating our
long chains of dead code, a simple solution for now seems fine.

This slightly changes the order of some optimizations, and so an opt_vpm
happens before opt_dce, causing 3 dead MOVs to be turned into dead FMAXes
in Minecraft:

instructions in affected programs:     52 -> 54 (3.85%)