vc4: Initialize undefined temporaries to 0.

vc4: Initialize undefined temporaries to 0.

Under the simulator, reading registers before writing them triggers an
assertion failure.  c->undef gets treated as r0, which will usually be
written, but not if it's used in the first instruction.  We should
definitely not be aborting in this case, and return some sort of undefined
value instead.

Fixes glsl-user-varying-ff.