panfrost/midgard: Refactor tag lookahead code

panfrost/midgard: Refactor tag lookahead code

Each Midgard instruction is scheduled to a particular instruction type
("tag"). Presumably the hardware prefetches memory based on tag, so it
is required to report out the first tag to the command stream and the
next tag of a branch target. This procedure was implemented in two
separate parts of the compiler (one time with a slight bug relating to
empty blocks); this patch refactors to unite the two routines and solve
the bug when branching to empty blocks.

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