LoongArch: Merge constant vector permuatation implementations.

There are currently two versions of the implementations of constant
vector permutation: loongarch_expand_vec_perm_const_1 and
loongarch_expand_vec_perm_const_2.  The implementations of the two
versions are different. Currently, only the implementation of
loongarch_expand_vec_perm_const_1 is used for 256-bit vectors.  We
hope to streamline the code as much as possible while retaining the
better-performing implementation of the two.  By repeatedly testing
spec2006 and spec2017, we got the following Merged version.
Compared with the pre-merger version, the number of lines of code
in loongarch.cc has been reduced by 888 lines.  At the same time,
the performance of SPECint2006 under Ofast has been improved by 0.97%,
and the performance of SPEC2017 fprate has been improved by 0.27%.

gcc/ChangeLog:

	* config/loongarch/loongarch.cc (loongarch_is_odd_extraction):
	Remove useless forward declaration.
	(loongarch_is_even_extraction): Remove useless forward declaration.
	(loongarch_try_expand_lsx_vshuf_const): Removed.
	(loongarch_expand_vec_perm_const_1): Merged.
	(loongarch_is_double_duplicate): Removed.
	(loongarch_is_center_extraction): Ditto.
	(loongarch_is_reversing_permutation): Ditto.
	(loongarch_is_di_misalign_extract): Ditto.
	(loongarch_is_si_misalign_extract): Ditto.
	(loongarch_is_lasx_lowpart_extract): Ditto.
	(loongarch_is_op_reverse_perm): Ditto.
	(loongarch_is_single_op_perm): Ditto.
	(loongarch_is_divisible_perm): Ditto.
	(loongarch_is_triple_stride_extract): Ditto.
	(loongarch_expand_vec_perm_const_2): Merged.
	(loongarch_expand_vec_perm_const): New.
	(loongarch_vectorize_vec_perm_const): Adjust.