LoongArch: Rework bswap{hi,si,di}2 definition

Per a gcc-help thread we are generating sub-optimal code for
__builtin_bswap{32,64}.  To fix it:

- Use a single revb.d instruction for bswapdi2.
- Use a single revb.2w instruction for bswapsi2 for TARGET_64BIT,
  revb.2h + rotri.w for !TARGET_64BIT.
- Use a single revb.2h instruction for bswapsi2 (x) r>> 16, and a single
  revb.2w instruction for bswapdi2 (x) r>> 32.

Unfortunately I cannot figure out a way to make the compiler generate
revb.4h or revh.{2w,d} instructions.

gcc/ChangeLog:

	* config/loongarch/loongarch.md (UNSPEC_REVB_2H, UNSPEC_REVB_4H,
	UNSPEC_REVH_D): Remove UNSPECs.
	(revb_4h, revh_d): Remove define_insn.
	(revb_2h): Define as (rotatert:SI (bswap:SI x) 16) instead of
	an UNSPEC.
	(revb_2h_extend, revb_2w, *bswapsi2, bswapdi2): New define_insn.
	(bswapsi2): Change to define_expand.  Only expand to revb.2h +
	rotri.w if !TARGET_64BIT.
	(bswapdi2): Change to define_insn of which the output is just a
	revb.d instruction.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/revb.c: New test.