nir/constant_folding: Add an unreachable to a switch

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/compiler/nir/nir_opt_constant_folding.c

@@ -89,6 +89,8 @@ constant_fold_alu_instr(nir_alu_instr *instr, void *mem_ctx)
          case 8:
             src[i].u8[j] = load_const->value.u8[instr->src[i].swizzle[j]];
             break;
+         default:
+            unreachable("Invalid bit size");
          }
       }