combine: Fix handling of unsigned constants

combine: Fix handling of unsigned constants

If a CONST_INT represents an integer of a mode with fewer bits than in
HOST_WIDE_INT, then the integer is sign extended.  For those two
optimizations touched by this patch, the integers of interest have only
the most significant bit set w.r.t their mode, therefore, they were sign
extended.  Thus in order to get the integer of interest, we have to chop
off the high bits.

gcc/ChangeLog:

	* combine.cc (simplify_compare_const): Fix handling of unsigned
	constants.