expand: Add dump for costing of positive divides

While trying to understand PR 115910 I found it was useful to print out
the two costs of doing a signed and unsigned division just like was added in
r15-3272-g3c89c41991d8e8 for popcount==1.

Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* expr.cc (expand_expr_divmod): Add dump of the two costs for
	positive division.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>