LoongArch: Avoid out-of-bounds access in loongarch_symbol_insns

We call loongarch_symbol_insns with mode = MAX_MACHINE_MODE sometimes.
But in loongarch_symbol_insns:

    if (LSX_SUPPORTED_MODE_P (mode) || LASX_SUPPORTED_MODE_P (mode))
      return 0;

And LSX_SUPPORTED_MODE_P is defined as:

    #define LSX_SUPPORTED_MODE_P(MODE) \
      (ISA_HAS_LSX \
       && GET_MODE_SIZE (MODE) == UNITS_PER_LSX_REG ... ...

GET_MODE_SIZE is expanded to a call to mode_to_bytes, which is defined:

    ALWAYS_INLINE poly_uint16
    mode_to_bytes (machine_mode mode)
    {
    #if GCC_VERSION >= 4001
      return (__builtin_constant_p (mode)
	  ? mode_size_inline (mode) : mode_size[mode]);
    #else
      return mode_size[mode];
    #endif
    }

There is an assertion in mode_size_inline:

    gcc_assert (mode >= 0 && mode < NUM_MACHINE_MODES);

Note that NUM_MACHINE_MODES = MAX_MACHINE_MODE (emitted by genmodes.cc),
thus if __builtin_constant_p (mode) is evaluated true (it happens when
GCC is bootstrapped with LTO+PGO), the assertion will be triggered and
cause an ICE.  OTOH if __builtin_constant_p (mode) is evaluated false,
mode_size[mode] is still an out-of-bound array access (the length or the
mode_size array is NUM_MACHINE_MODES).

So we shouldn't call LSX_SUPPORTED_MODE_P or LASX_SUPPORTED_MODE_P with
MAX_MACHINE_MODE in loongarch_symbol_insns.  This is very similar to a
MIPS bug PR98491 fixed by me about 3 years ago.

gcc/ChangeLog:

	* config/loongarch/loongarch.cc (loongarch_symbol_insns): Do not
	use LSX_SUPPORTED_MODE_P or LASX_SUPPORTED_MODE_P if mode is
	MAX_MACHINE_MODE.