vect-patterns: Fix up vect_widened_op_tree [PR108692]

The following testcase is miscompiled on aarch64-linux since r11-5160.
Given
  <bb 3> [local count: 955630225]:
  # i_22 = PHI <i_20(6), 0(5)>
  # r_23 = PHI <r_19(6), 0(5)>
...
  a.0_5 = (unsigned char) a_15;
  _6 = (int) a.0_5;
  b.1_7 = (unsigned char) b_17;
  _8 = (int) b.1_7;
  c_18 = _6 - _8;
  _9 = ABS_EXPR <c_18>;
  r_19 = _9 + r_23;
...
where SSA_NAMEs 15/17 have signed char, 5/7 unsigned char and rest is int
we first pattern recognize c_18 as
patt_34 = (a.0_5) w- (b.1_7);
which is still correct, 5/7 are unsigned char subtracted in wider type,
but then vect_recog_sad_pattern turns it into
SAD_EXPR <a_15, b_17, r_23>
which is incorrect, because 15/17 are signed char and so it is
sum of absolute signed differences rather than unsigned sum of
absolute unsigned differences.
The reason why this happens is that vect_recog_sad_pattern calls
vect_widened_op_tree with MINUS_EXPR, WIDEN_MINUS_EXPR on the
patt_34 = (a.0_5) w- (b.1_7); statement's vinfo and vect_widened_op_tree
calls vect_look_through_possible_promotion on the operands of the
WIDEN_MINUS_EXPR, which looks through the further casts.
vect_look_through_possible_promotion has careful code to stop when there
would be nested casts that need to be preserved, but the problem here
is that the WIDEN_*_EXPR operation itself has an implicit cast on the
operands already - in this case of WIDEN_MINUS_EXPR the unsigned char
5/7 SSA_NAMEs are widened to unsigned short before the subtraction,
and vect_look_through_possible_promotion obviously isn't told about that.

Now, I think when we see those WIDEN_{MULT,MINUS,PLUS}_EXPR codes, we had
to look through possible promotions already when creating those and so
vect_look_through_possible_promotion again isn't really needed, all we need
to do is arrange what that function will do if the operand isn't result
of any cast.  Other option would be let vect_look_through_possible_promotion
know about the implicit promotion from the WIDEN_*_EXPR, but I'm afraid
that would be much harder.

2023-02-08  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/108692
	* tree-vect-patterns.cc (vect_widened_op_tree): If rhs_code is
	widened_code which is different from code, don't call
	vect_look_through_possible_promotion but instead just check op is
	SSA_NAME with integral type for which vect_is_simple_use is true
	and call set_op on this_unprom.

	* gcc.dg/pr108692.c: New test.