pan/bi: Implement store_output for fragment shaders

Corresponds to a BLEND instruction, possibly preceded by an ATEST
instruction.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4097>
diff --git a/src/panfrost/bifrost/compiler.h b/src/panfrost/bifrost/compiler.h
index 71d1b24..cbd76d5 100644
--- a/src/panfrost/bifrost/compiler.h
+++ b/src/panfrost/bifrost/compiler.h
@@ -330,6 +330,7 @@
        bi_block *after_block;
        bi_block *break_block;
        bi_block *continue_block;
+       bool emitted_atest;
 
        /* Stats for shader-db */
        unsigned instruction_count;