LoongArch: Use simplify_gen_subreg instead of gen_rtx_SUBREG in loongarch_expand_vec_...

LoongArch: Use simplify_gen_subreg instead of gen_rtx_SUBREG in loongarch_expand_vec_cond_mask_expr [PR112476]

GCC internal says:

    'subreg's of 'subreg's are not supported.  Using
    'simplify_gen_subreg' is the recommended way to avoid this problem.

Unfortunately loongarch_expand_vec_cond_mask_expr might create nested
subreg under certain circumstances, causing an ICE.

Use simplify_gen_subreg as the internal document suggests.

gcc/ChangeLog:

	PR target/112476
	* config/loongarch/loongarch.cc
	(loongarch_expand_vec_cond_mask_expr): Call simplify_gen_subreg
	instead of gen_rtx_SUBREG.

gcc/testsuite/ChangeLog:

	PR target/112476
	* gcc.target/loongarch/pr112476-1.c: New test.
	* gcc.target/loongarch/pr112476-2.c: New test.