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_shader.cpp

@@ -453,8 +453,8 @@ brw_instruction_name(const struct gen_device_info *devinfo, enum opcode op)
    case FS_OPCODE_PACK_HALF_2x16_SPLIT:
       return "pack_half_2x16_split";
 
-   case FS_OPCODE_PLACEHOLDER_HALT:
-      return "placeholder_halt";
+   case SHADER_OPCODE_HALT_TARGET:
+      return "halt_target";
 
    case FS_OPCODE_INTERPOLATE_AT_SAMPLE:
       return "interp_sample";