vc4: Update copy propagation for control flow.

vc4: Update copy propagation for control flow.

Previously, we could assume that a MOV from a temp was always an available
copy, because all temps were SSA in NIR, and their non-SSA state in QIR
was just due to the fact that they were from a bcsel or pack_unorm_4x8, so
we could use the current value of the temp after that series of QIR
instructions to define it.

However, this is no longer the case with control flow.  Instead, we track
a new array of MOVs defined within the block that haven't had their source
or dest killed yet, and use that primarily.  We fall back to looking
through the QIR defs array to handle across-block MOVs, but now require
that copies from the SSA defs have an SSA src as well.