pan/bi: Fix vector handling of readmasks

pan/bi: Fix vector handling of readmasks

The issue was messing with liveness analysis... with Midgard we look at
the writemask to decide how the instruction behaves. Here, since our ALU
is scalar (except for subdivision which doesn't have proper writemasks
anyway) we just look at the component count directly -- either 4 for
vector instructions (essentially - for smaller loads we can replicate
manually without much burden), or 1 for scalar.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4158>