Mitigate -Wmaybe-uninitialized in expmed.cc.

Mitigate -Wmaybe-uninitialized in expmed.cc.

It's the warning I see every time I build GCC:

In file included from /home/marxin/Programming/gcc/gcc/coretypes.h:478,
                 from /home/marxin/Programming/gcc/gcc/expmed.cc:26:
In function ‘poly_uint16 mode_to_bytes(machine_mode)’,
    inlined from ‘typename if_nonpoly<typename T::measurement_type>::type GET_MODE_SIZE(const T&) [with T = scalar_int_mode]’ at /home/marxin/Programming/gcc/gcc/machmode.h:647:24,
    inlined from ‘rtx_def* emit_store_flag_1(rtx, rtx_code, rtx, rtx, machine_mode, int, int, machine_mode)’ at /home/marxin/Programming/gcc/gcc/expmed.cc:5728:56:
/home/marxin/Programming/gcc/gcc/machmode.h:550:49: warning: ‘*(unsigned int*)((char*)&int_mode + offsetof(scalar_int_mode, scalar_int_mode::m_mode))’ may be used uninitialized [-Wmaybe-uninitialized]
  550 |           ? mode_size_inline (mode) : mode_size[mode]);
      |                                                 ^~~~
/home/marxin/Programming/gcc/gcc/expmed.cc: In function ‘rtx_def* emit_store_flag_1(rtx, rtx_code, rtx, rtx, machine_mode, int, int, machine_mode)’:
/home/marxin/Programming/gcc/gcc/expmed.cc:5657:19: note: ‘*(unsigned int*)((char*)&int_mode + offsetof(scalar_int_mode, scalar_int_mode::m_mode))’ was declared here
 5657 |   scalar_int_mode int_mode;
      |                   ^~~~~~~~

Can we please mitigate it?

gcc/ChangeLog:

	* expmed.cc (emit_store_flag_1): Mitigate -Wmaybe-uninitialized
	warning.