agx: Track logical control flow graph

Logic lifted from the Bifrost compiler, which was a copypaste of the Midgard
compiler, which was based on a faulty understanding of the v3d compiler,
which...

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11718>

src/asahi/compiler/agx_compiler.h

@@ -364,6 +364,9 @@ typedef struct {
 
    /* During instruction selection, for inserting control flow */
    agx_block *current_block;
+   agx_block *continue_block;
+   agx_block *break_block;
+   agx_block *after_block;
 
    /* Stats for shader-db */
    unsigned loop_count;