rs6000: Remove unspecs for vec_mrghl[bhw]

rs6000: Remove unspecs for vec_mrghl[bhw]

vmrghb only accepts permute index {0, 16, 1, 17, 2, 18, 3, 19, 4, 20,
5, 21, 6, 22, 7, 23} no matter for BE or LE in ISA, similarly for vmrglb.
Remove UNSPEC_VMRGH_DIRECT/UNSPEC_VMRGL_DIRECT pattern as vec_select
+ vec_concat as normal RTL.

Tested pass on P8LE, P9LE and P8BE{m32}.

gcc/ChangeLog:

2021-10-19  Xionghu Luo  <luoxhu@linux.ibm.com>

	* config/rs6000/altivec.md (*altivec_vmrghb_internal): Delete.
	(altivec_vmrghb_direct): New.
	(*altivec_vmrghh_internal): Delete.
	(altivec_vmrghh_direct): New.
	(*altivec_vmrghw_internal): Delete.
	(altivec_vmrghw_direct_<mode>): New.
	(altivec_vmrghw_direct): Delete.
	(*altivec_vmrglb_internal): Delete.
	(altivec_vmrglb_direct): New.
	(*altivec_vmrglh_internal): Delete.
	(altivec_vmrglh_direct): New.
	(*altivec_vmrglw_internal): Delete.
	(altivec_vmrglw_direct_<mode>): New.
	(altivec_vmrglw_direct): Delete.
	* config/rs6000/rs6000-p8swap.c (rtx_is_swappable_p): Adjust.
	* config/rs6000/rs6000.c (altivec_expand_vec_perm_const):
	Adjust.
	* config/rs6000/vsx.md (vsx_xxmrghw_<mode>): Adjust.
	(vsx_xxmrglw_<mode>): Adjust.

gcc/testsuite/ChangeLog:

2021-10-19  Xionghu Luo  <luoxhu@linux.ibm.com>

	* gcc.target/powerpc/builtins-1.c: Update instruction counts.