pan/midgard: Report byte masks for read components

pan/midgard: Report byte masks for read components

Read component masks don't have a particular type associated, since the
type of the ALU operation may not match the type of the operands in
question. So let's generate byte masks instead, and update the rest of
the compiler to use byte masks when analyzing reads.

Preparation for mixed types.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>