panfrost: Dynamically set discard branch targets

panfrost: Dynamically set discard branch targets

discard and discard_if are both implemented with the branching pipeline
on Midgard; essentially, we branch to the end of the fragment shader in
a special "discard" mode, setting the condition as necessary.
Previously, we hardcoded the form of this instruction, which worked for
very simple shaders but was incorrect for anything remotely interesting.
This patch instead emits logical branches in the IR, which are flattened
to real discard ops the same way other branches are, allowing targets to
be computed correctly.

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