pan/bi: Fix 'last tuple' for terminal-NOP clauses

This incorrectly assumed the last tuple was nontrivial, an invariant
that holds by accident on the current scheduler but will break
momentarily. Fix this, and document the invariants.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8962>

src/panfrost/bifrost/compiler.h

@@ -840,18 +840,33 @@ bi_after_instr(bi_instr *instr)
     };
 }
 
-static inline bi_instr *
+/* Invariant: a tuple must be nonempty UNLESS it is the last tuple of a clause,
+ * in which case there must exist a nonempty penultimate tuple */
+
+ATTRIBUTE_RETURNS_NONNULL static inline bi_instr *
 bi_first_instr_in_clause(bi_clause *clause)
 {
         bi_tuple tuple = clause->tuples[0];
-        return tuple.fma ?: tuple.add;
+        bi_instr *instr = tuple.fma ?: tuple.add;
+
+        assert(instr != NULL);
+        return instr;
 }
 
-static inline bi_instr *
+ATTRIBUTE_RETURNS_NONNULL static inline bi_instr *
 bi_last_instr_in_clause(bi_clause *clause)
 {
         bi_tuple tuple = clause->tuples[clause->tuple_count - 1];
-        return tuple.add ?: tuple.fma;
+        bi_instr *instr = tuple.add ?: tuple.fma;
+
+        if (!instr) {
+                assert(clause->tuple_count >= 2);
+                tuple = clause->tuples[clause->tuple_count - 2];
+                instr = tuple.add ?: tuple.fma;
+        }
+
+        assert(instr != NULL);
+        return instr;
 }
 
 /* Implemented by expanding bi_foreach_instr_in_block_from(_rev) with the start