pan/midgard: Add helpers for manipulating byte masks

pan/midgard: Add helpers for manipulating byte masks

There are essentially two formats of masks in play beginning with this
commit: masks per-channel and masks per-byte. The former make sense
within a given fixed-size instruction; the latter are
typesize-independent. It turns out you need the latter to meaningfully
manipulate instructions containing multiple sizes (which is quite
possible with ALU operations).

Similarly, we have mir_srcsize. We calculate the size of the source by
analyzing the size of the instruction itself and stepping down if there
is a half-modifier.

Finally, we have mir_round_bytemask_down, for when we want to take a
byte mask and "round it down" to a given component size, so that we can
use it as a component mask.

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