rtl-optimization/110939 Really fix narrow comparison of memory and constant

rtl-optimization/110939 Really fix narrow comparison of memory and constant

In the former fix in commit 41ef5a34161356817807be3a2e51fbdbe575ae85 I
completely missed the fact that the normal form of a CONST_INT for a
mode with fewer bits than in HOST_WIDE_INT is a sign extended version of
the actual constant.  This even holds true for unsigned constants.

Fixed by masking out the upper bits for the incoming constant and sign
extending the resulting unsigned constant.

gcc/ChangeLog:

	* combine.cc (simplify_compare_const): Properly handle unsigned
	constants while narrowing comparison of memory and constants.