vc4: Match VS outputs to FS inputs.

vc4: Match VS outputs to FS inputs.

If the VS doesn't output a value that the FS needs, we still need to read
the right contents for the remaining FS inputs, by emitting padding.  And
if the VS outputs something the FS doesn't need, we shouldn't put it in
the VPM at all (so the code producing it can get DCEed).

Fixes 77 piglit tests.