InstCombine: Don't unconditionally preserve 'nuw' when shrinking constants

Consider:
  %add = add nuw i32 %a, -16777216
  %and = and i32 %add, 255

Regardless of whether or not we demand the sign bit of %add, we cannot
replace -16777216 with 2130706432 without also removing 'nuw' from the
instruction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216273 91177308-0d34-0410-b5e6-96231b3b80d8