freedreno/ir3: handle "holes" in inputs

freedreno/ir3: handle "holes" in inputs

If, for example, only the x/y/w components of in.xyzw are actually used,
we still need to have a group of four registers and assign all four
components.  The hardware can't write in.xy and in.w to discontiguous
registers.  To handle this, pad with a dummy NOP instruction, to keep
the neighbor chain contiguous.

This fixes a problem noticed with firefox OMTC.

Signed-off-by: Rob Clark <robclark@freedesktop.org>