vc4: Fix live intervals analysis for screening defs in if statements.

vc4: Fix live intervals analysis for screening defs in if statements.

If a conditional assignment is only conditioned on the exec mask, that's
still screening off the value in the executed channels (and, since we're
not storing to the unexcuted channels, we don't care what's in there).

Fixes a bunch of extra register pressure on Processing's Ribbons demo,
which is failing to allocate.