vectorizer: Fix up vectorization using WIDEN_MINUS_EXPR [PR102124]

The following testcase is miscompiled on aarch64-linux at -O3 since the
introduction of WIDEN_MINUS_EXPR.
The problem is if the inner type (half_type) is unsigned and the result
type in which the subtraction is performed (type) has precision more than
twice as larger as the inner type's precision.
For other widening operations like WIDEN_{PLUS,MULT}_EXPR, if half_type
is unsigned, the addition/multiplication result in itype is also unsigned
and needs to be zero-extended to type.
But subtraction is special, even when half_type is unsigned, the subtraction
behaves as signed (also regardless of whether the result type is signed or
unsigned), 0xfeU - 0xffU is -1 or 0xffffffffU, not 0x0000ffff.

I think it is better not to use mixed signedness of types in
WIDEN_MINUS_EXPR (have unsigned vector of operands and signed result
vector), so this patch instead adds another cast to make sure we always
sign-extend the result from itype to type if type is wider than itype.

2021-09-01  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/102124
	* tree-vect-patterns.c (vect_recog_widen_op_pattern): For ORIG_CODE
	MINUS_EXPR, if itype is unsigned with smaller precision than type,
	add an extra cast to signed variant of itype to ensure sign-extension.

	* gcc.dg/torture/pr102124.c: New test.