widening_mul: Fix up ICE caused by my signed multiplication overflow pattern recognit...

widening_mul: Fix up ICE caused by my signed multiplication overflow pattern recognition changes [PR98629]

As the testcase shows, my latest changes caused ICE on that testcase.
The problem is that arith_overflow_check_p now can change the use_stmt
argument (has a reference), so that if it succeeds (returns non-zero),
it points it to the GIMPLE_COND or EQ/NE or COND_EXPR assignment from the
TRUNC_DIV_EXPR assignment.
The problem was that it would change use_stmt also if it returned 0 in some
cases, such as multiple imm uses of the division, and in one of the callers
if arith_overflow_check_p returns 0 it looks at use_stmt again and performs
other checks, which of course assumes that use_stmt is the one passed
to arith_overflow_check_p and not e.g. NULL instead or some other unrelated
stmt.

The following patch fixes that by only changing use_stmt when we are about
to return non-zero (for the MULT_EXPR case, which is the only one with the
need to use different use_stmt).

2021-01-12  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/98629
	* tree-ssa-math-opts.c (arith_overflow_check_p): Don't update use_stmt
	unless returning non-zero.

	* gcc.c-torture/compile/pr98629.c: New test.