LoongArch: Add bit reverse operations

LoongArch: Add bit reverse operations

LoongArch supports native bit reverse operation for QI, SI, DI, and for
HI we can expand it into a shift and a bit reverse in word_mode.

I was reluctant to add them because until PR50481 is fixed these
operations will be just useless.  But now it turns out we can use them
to optimize the bit reversing CRC calculation if recognized by the
generic CRC pass.  So add them in prepare for the next patch adding CRC
expanders.

gcc/ChangeLog:

	* config/loongarch/loongarch.md (@rbit<mode:GPR>): New
	define_insn template.
	(rbitsi_extended): New define_insn.
	(rbitqi): New define_insn.
	(rbithi): New define_expand.