host_amd64_defs.c don't initialize opc and subopc_imm in emit_AMD64Instr.

host_amd64_defs.c don't initialize opc and subopc_imm in emit_AMD64Instr.

In the case of Ain_SseShiftN we first assign zero to opc and subopc_imm
before handling the various subops. But since we will (and must) always
assign a valid value to opc and subopc_imm we might get a compiler warning
about the values never being read before storing a different value.

So explicitly don't assign a value. Then the compiler will warn if we
would ever forget to assign it a value value later on before using it.