forwprop: Fix underlying type of long_mul_kind and long_mul_extract

Fixing the underyling type can safe some space slightly.  Since
these enums are less than 8bit in its usage fix the underlying type
to unsigned char.

Pushed as obvious after bootstrap/test on x86_64-linux-gnu.

gcc/ChangeLog:

	* tree-ssa-forwprop.cc (enum long_mul_kind): Fix to unsigned char.
	(enum long_mul_extract): Likewise.

Signed-off-by: Andrea Pinski <andrew.pinski@oss.qualcomm.com>