LoongArch: Fix ICE and use simplify_gen_subreg instead of gen_rtx_SUBREG directly.

LoongArch: Fix ICE and use simplify_gen_subreg instead of gen_rtx_SUBREG directly.

loongarch_expand_vec_cond_mask_expr generates 'subreg's of 'subreg's, which are not supported
in gcc, it causes an ICE:

ice.c:55:1: error: unrecognizable insn:
   55 | }
      | ^
(insn 63 62 64 8 (set (reg:V4DI 278)
        (subreg:V4DI (subreg:V4DF (reg:V4DI 273 [ vect__53.26 ]) 0) 0)) -1
     (nil))
during RTL pass: vregs
ice.c:55:1: internal compiler error: in extract_insn, at recog.cc:2804

Last time, Ruoyao has fixed a similar ICE:
https://gcc.gnu.org/pipermail/gcc-patches/2023-November/636156.html

This patch fixes ICE and use simplify_gen_subreg instead of gen_rtx_SUBREG as much as possible
to avoid the same ice happening again.

gcc/ChangeLog:

	* config/loongarch/loongarch.cc (loongarch_try_expand_lsx_vshuf_const): Use
	simplify_gen_subreg instead of gen_rtx_SUBREG.
	(loongarch_expand_vec_perm_const_2): Ditto.
	(loongarch_expand_vec_cond_expr): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/pr112476-3.c: New test.
	* gcc.target/loongarch/pr112476-4.c: New test.