spirv: add switch case for nir_texop_txf_ms_mcs in vtn_handle_texture()

Mark it as unreachable.  Silences a compiler warning:

spirv/spirv_to_nir.c:1397:4: warning: enumeration value
'nir_texop_txf_ms_mcs' not handled in switch [-Wswitch]
    switch (instr->op) {
    ^

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>

src/compiler/spirv/spirv_to_nir.c

@@ -1413,6 +1413,8 @@ vtn_handle_texture(struct vtn_builder *b, SpvOp opcode,
       /* These don't */
       instr->sampler = NULL;
       break;
+   case nir_texop_txf_ms_mcs:
+      unreachable("unexpected nir_texop_txf_ms_mcs");
    }
 
    nir_ssa_dest_init(&instr->instr, &instr->dest,