widening_mul: Fix up signed multiplication overflow check handling [PR98727]

widening_mul: Fix up signed multiplication overflow check handling [PR98727]

I forgot one line, which means that if the second operand of the multiplication
isn't constant, it would be just the same as the first one.

2021-01-18  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/98727
	* tree-ssa-math-opts.c (match_arith_overflow): Fix up computation of
	second .MUL_OVERFLOW operand for signed multiplication with overflow
	checking if the second operand of multiplication is not constant.

	* gcc.c-torture/execute/pr98727.c: New test.