rs6000: Fix wrong RTL patterns for vector merge high/low char on LE

rs6000: Fix wrong RTL patterns for vector merge high/low char on LE

Commit r12-4496 changes some define_expands and define_insns
for vector merge high/low char, which are altivec_vmrg[hl]b.
These defines are mainly for built-in function vec_merge{h,l}
and some internal gen function needs.  These functions should
consider endianness, taking vec_mergeh as example, as PVIPR
defines, vec_mergeh "Merges the first halves (in element order)
of two vectors", it does note it's in element order.  So it's
mapped into vmrghb on BE while vmrglb on LE respectively.
Although the mapped insns are different, as the discussion in
PR106069, the RTL pattern should be still the same, it is
conformed before commit r12-4496, but gets changed into
different patterns on BE and LE starting from commit r12-4496.
Similar to 32-bit element case in commit log of r15-1504, this
8-bit element pattern on LE doesn't actually match what the
underlying insn is intended to represent, once some optimization
like combine does some changes basing on it, it would cause
the unexpected consequence.  The newly constructed test case
pr106069-1.c is a typical example for this issue.

So this patch is to fix the wrong RTL pattern, ensure the
associated RTL patterns become the same as before which can
have the same semantic as their mapped insns.  With the
proposed patch, the expanders like altivec_vmrghb expands
into altivec_vmrghb_direct_be or altivec_vmrglb_direct_le
depending on endianness, "direct" can easily show which
insn would be generated, _be and _le are mainly for the
different RTL patterns as endianness.

Co-authored-by: Xionghu Luo <xionghuluo@tencent.com>
	PR target/106069
	PR target/115355

gcc/ChangeLog:

	* config/rs6000/altivec.md (altivec_vmrghb_direct): Rename to ...
	(altivec_vmrghb_direct_be): ... this.  Add condition BYTES_BIG_ENDIAN.
	(altivec_vmrghb_direct_le): New define_insn.
	(altivec_vmrglb_direct): Rename to ...
	(altivec_vmrglb_direct_be): ... this.  Add condition BYTES_BIG_ENDIAN.
	(altivec_vmrglb_direct_le): New define_insn.
	(altivec_vmrghb): Adjust by calling gen_altivec_vmrghb_direct_be
	for BE and gen_altivec_vmrglb_direct_le for LE.
	(altivec_vmrglb): Adjust by calling gen_altivec_vmrglb_direct_be
	for BE and gen_altivec_vmrghb_direct_le for LE.
	* config/rs6000/rs6000.cc (altivec_expand_vec_perm_const): Replace
	CODE_FOR_altivec_vmrghb_direct by
	CODE_FOR_altivec_vmrghb_direct_be for BE and
	CODE_FOR_altivec_vmrghb_direct_le for LE.  And replace
	CODE_FOR_altivec_vmrglb_direct by
	CODE_FOR_altivec_vmrglb_direct_be for BE and
	CODE_FOR_altivec_vmrglb_direct_le for LE.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/pr106069-1.c: New test.