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

Commit r12-4496 changes some define_expands and define_insns
for vector merge high/low word, which are altivec_vmrg[hl]w,
vsx_xxmrg[hl]w_<VSX_W:mode>.  These defines are mainly for
built-in function vec_merge{h,l}, __builtin_vsx_xxmrghw,
__builtin_vsx_xxmrghw_4si 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 vmrghw on
BE while vmrglw 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, define_expand altivec_vmrghw got expanded
into:

  (vec_select:VSX_W
     (vec_concat:<VS_double>
        (match_operand:VSX_W 1 "register_operand" "wa,v")
        (match_operand:VSX_W 2 "register_operand" "wa,v"))
        (parallel [(const_int 0) (const_int 4)
                   (const_int 1) (const_int 5)])))]

on both BE and LE then.  But commit r12-4496 changed it to
expand into:

  (vec_select:VSX_W
     (vec_concat:<VS_double>
        (match_operand:VSX_W 1 "register_operand" "wa,v")
        (match_operand:VSX_W 2 "register_operand" "wa,v"))
        (parallel [(const_int 0) (const_int 4)
                   (const_int 1) (const_int 5)])))]

on BE, and

  (vec_select:VSX_W
     (vec_concat:<VS_double>
        (match_operand:VSX_W 1 "register_operand" "wa,v")
        (match_operand:VSX_W 2 "register_operand" "wa,v"))
        (parallel [(const_int 2) (const_int 6)
                   (const_int 3) (const_int 7)])))]

on LE, although the mapped insn are still vmrghw on BE and
vmrglw on LE, the associated RTL pattern is completely
wrong and inconsistent with the mapped insn.  If optimization
passes leave this pattern alone, even if its pattern doesn't
represent its mapped insn, it's still fine, that's why simple
testing on bif doesn't expose this issue.  But once some
optimization pass such as combine does some changes basing
on this wrong pattern, because the pattern doesn't match the
semantics that the expanded insn is intended to represent,
it would cause the unexpected result.

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_vmrghw 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_vmrghw_direct_<VSX_W:mode>): Rename
	to ...
	(altivec_vmrghw_direct_<VSX_W:mode>_be): ... this.  Add the condition
	BYTES_BIG_ENDIAN.
	(altivec_vmrghw_direct_<VSX_W:mode>_le): New define_insn.
	(altivec_vmrglw_direct_<VSX_W:mode>): Rename to ...
	(altivec_vmrglw_direct_<VSX_W:mode>_be): ... this.  Add the condition
	BYTES_BIG_ENDIAN.
	(altivec_vmrglw_direct_<VSX_W:mode>_le): New define_insn.
	(altivec_vmrghw): Adjust by calling gen_altivec_vmrghw_direct_v4si_be
	for BE and gen_altivec_vmrglw_direct_v4si_le for LE.
	(altivec_vmrglw): Adjust by calling gen_altivec_vmrglw_direct_v4si_be
	for BE and gen_altivec_vmrghw_direct_v4si_le for LE.
	(vec_widen_umult_hi_v8hi): Adjust the call to
	gen_altivec_vmrghw_direct_v4si by gen_altivec_vmrghw for BE
	and by gen_altivec_vmrglw for LE.
	(vec_widen_smult_hi_v8hi): Likewise.
	(vec_widen_umult_lo_v8hi): Adjust the call to
	gen_altivec_vmrglw_direct_v4si by gen_altivec_vmrglw for BE
	and by gen_altivec_vmrghw for LE
	(vec_widen_smult_lo_v8hi): Likewise.
	* config/rs6000/rs6000.cc (altivec_expand_vec_perm_const): Replace
	CODE_FOR_altivec_vmrghw_direct_v4si by
	CODE_FOR_altivec_vmrghw_direct_v4si_be for BE and
	CODE_FOR_altivec_vmrghw_direct_v4si_le for LE.  And replace
	CODE_FOR_altivec_vmrglw_direct_v4si by
	CODE_FOR_altivec_vmrglw_direct_v4si_be for BE and
	CODE_FOR_altivec_vmrglw_direct_v4si_le for LE.
	* config/rs6000/vsx.md (vsx_xxmrghw_<VSX_W:mode>): Adjust by calling
	gen_altivec_vmrghw_direct_v4si_be for BE and
	gen_altivec_vmrglw_direct_v4si_le for LE.
	(vsx_xxmrglw_<VSX_W:mode>): Adjust by calling
	gen_altivec_vmrglw_direct_v4si_be for BE and
	gen_altivec_vmrghw_direct_v4si_le for LE.

gcc/testsuite/ChangeLog:

	* g++.target/powerpc/pr106069.C: New test.
	* gcc.target/powerpc/pr115355.c: New test.

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