nir/peephole_select: allow mbcnt_amd

It's just alu, so handle it like alu.

Foz-DB Navi21:
Totals from 3 (0.00% of 97591) affected shaders:
Instrs: 433 -> 426 (-1.62%)
CodeSize: 2408 -> 2388 (-0.83%)
Latency: 7520 -> 7925 (+5.39%)
InvThroughput: 857 -> 1009 (+17.74%)
Copies: 55 -> 43 (-21.82%)
Branches: 21 -> 17 (-19.05%)
SALU: 79 -> 76 (-3.80%)

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38828>

src/compiler/nir/nir_opt_peephole_select.c

@@ -202,6 +202,7 @@ block_check_for_allowed_instrs(nir_block *block, unsigned *count,
          case nir_intrinsic_load_const_ir3:
          case nir_intrinsic_ballot:
          case nir_intrinsic_ballot_relaxed:
+         case nir_intrinsic_mbcnt_amd:
             if (!alu_ok)
                return false;
             break;