nir: Pick just the channels we want for bitmap and drawpixels lowering.

nir: Pick just the channels we want for bitmap and drawpixels lowering.

NIR now validates that SSA references use the same number of channels as
are in the SSA value.

v2: Reword commit message, since the commit didn't land before the
    validation change did.

Fixes: 370d68babcbb ("nir/validate: Validate that bit sizes and components always match")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net> (v1)
Cc: <mesa-stable@lists.freedesktop.org>