intel/brw: Clear write_accumulator flag when changing the destination

If the destination was the accumulator but is no longer, having the flag
set is not correct. On Xe2 this also causes a validation error.

v2: Reword the comment to be more clear. Suggested by Jordan.

Fixes: efa4e4bc5f ("intel/fs: Introduce regioning lowering pass.")
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28404>

src/intel/compiler/brw_fs_lower_regioning.cpp

@@ -550,6 +550,12 @@ namespace {
          ibld.at(block, inst->next).MOV(subscript(inst->dst, raw_type, j),
                                         subscript(tmp, raw_type, j));
 
+      /* If the destination was an accumulator, after lowering it will be a
+       * GRF. Clear writes_accumulator for the instruction.
+       */
+      if (inst->dst.is_accumulator())
+         inst->writes_accumulator = false;
+
       /* Point the original instruction at the temporary, making sure to keep
        * any destination modifiers in the instruction.
        */