LoongArch: Combine xor and crc instructions

For a textbook-style CRC implementation:

    uint32_t crc = 0xffffffffu;
    for (size_t k = 0; k < len; k++)
      {
	crc ^= data[k];
	for (int i = 0; i < 8 * sizeof (T); i++)
	  if (crc & 1)
	    crc = (crc >> 1) ^ poly;
	  else
	    crc >>= 1;
      }
    return crc;

The generic code reports:

    Data and CRC are xor-ed before for loop.  Initializing data with 0.

resulting in:

    ld.bu     $t1, $a0, 0
    xor       $t0, $t0, $t1
    crc.w.b.w $t0, $zero, $t0

But it's just better to use

    ld.bu     $t1, $a0, 0
    crc.w.b.w $t0, $t1, $t0

instead.  Implement this optimization now.

gcc/ChangeLog:

	* config/loongarch/loongarch.md (*crc_combine): New
	define_insn_and_split.

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