intel/fs: Rename PLACEHOLDER_HALT to HALT_TARGET

It's a bit more explicit and will play more nicely with what we're about
to do.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5071>

src/intel/compiler/brw_ir_performance.cpp

@@ -1098,7 +1098,7 @@ namespace {
          }
 
       case SHADER_OPCODE_UNDEF:
-      case FS_OPCODE_PLACEHOLDER_HALT:
+      case SHADER_OPCODE_HALT_TARGET:
       case FS_OPCODE_SCHEDULING_FENCE:
          return calculate_desc(info, unit_null, 0, 0, 0, 0, 0,
                                0, 0, 0, 0, 0, 0);
@@ -1552,7 +1552,7 @@ namespace {
 
             issue_instruction(st, s->devinfo, inst);
 
-            if (inst->opcode == FS_OPCODE_PLACEHOLDER_HALT && discard_count)
+            if (inst->opcode == SHADER_OPCODE_HALT_TARGET && discard_count)
                st.weight /= discard_weight;
 
             elapsed += (st.unit_ready[unit_fe] - clock0) * st.weight;