pan/midgard: Add scheduling barriers

pan/midgard: Add scheduling barriers

Scheduling occurs on a per-block basis, strongly assuming that a given
block contains at most a single branch. This does not always map to the
source NIR control flow, particularly when discard intrinsics are
involved. The solution is to allow scheduling barriers, which will
terminate a block early in code generation and open a new block.

To facilitate this, we need to move some post-block processing to a new
pass, rather than relying hackily on the current_block pointer.

This allows us to cleanup some logic analyzing branches in other parts
of the driver us well, now that the MIR is much more well-formed.

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