pan/midgard: Use better heuristic for shader termination

pan/midgard: Use better heuristic for shader termination

This still may not be perfect (in the sense that legal shaders might
still get cut off) but this fits how writeout is done with both Panfrost
and the blob, so it's good enough for what we need and allows MRT
shaders to be sanely disassembled.

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