ir3: fix predicate splitting in scheduler

Fix up src->def->instr, not src->instr.

Cc: mesa-stable
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/7014
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18096>
(cherry picked from commit b1cb764316)

.pick_status.json

@@ -9751,7 +9751,7 @@
         "description": "ir3: fix predicate splitting in scheduler",
         "nominated": true,
         "nomination_type": 0,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": null
     },

src/freedreno/ir3/ir3.h

@@ -196,8 +196,8 @@ struct ir3_register {
       } array;
    };
 
-   /* For IR3_REG_DEST, pointer back to the instruction containing this
-    * register.
+   /* For IR3_REG_SSA, dst registers contain pointer back to the instruction
+    * containing this register.
     */
    struct ir3_instruction *instr;
 

src/freedreno/ir3/ir3_sched.c

@@ -992,7 +992,7 @@ split_pred(struct ir3_sched_ctx *ctx)
             /* original pred is scheduled, but new one isn't: */
             new_pred->flags &= ~IR3_INSTR_MARK;
          }
-         predicated->srcs[0]->instr = new_pred;
+         predicated->srcs[0]->def->instr = new_pred;
          /* don't need to remove old dag edge since old pred is
           * already scheduled:
           */