intel/compiler: Implement nir_intrinsic_last_invocation

We haven't exposed this intrinsic as it doesn't directly correspond to
anything in SPIR-V.  However, it's used internally by some NIR passes,
namely nir_opt_uniform_atomics().

We reuse most of the infrastructure in brw_find_live_channel, but with
LZD/ADD instead of FBL.  A new SHADER_OPCODE_FIND_LAST_LIVE_CHANNEL is
like SHADER_OPCODE_FIND_LIVE_CHANNEL but from the other side.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15484>

src/intel/compiler/brw_shader.cpp

@@ -384,6 +384,8 @@ brw_instruction_name(const struct intel_device_info *devinfo, enum opcode op)
 
    case SHADER_OPCODE_FIND_LIVE_CHANNEL:
       return "find_live_channel";
+   case SHADER_OPCODE_FIND_LAST_LIVE_CHANNEL:
+      return "find_last_live_channel";
    case FS_OPCODE_LOAD_LIVE_CHANNELS:
       return "load_live_channels";