[InstCombine] Use a more targeted fix instead of r235544

Only clear out the NSW/NUW flags if we are optimizing 'add'/'sub' while
taking advantage that the sign bit is not set.  We do this optimization
to further shrink the mask but shrinking the mask isn't NSW/NUW
preserving in this case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235558 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/InstCombine/cast.ll

@@ -1125,3 +1125,15 @@ define i1 @PR23309(i32 %A, i32 %B) {
   %trunc = trunc i32 %sub to i1
   ret i1 %trunc
 }
+
+define i1 @PR23309v2(i32 %A, i32 %B) {
+; CHECK-LABEL: @PR23309v2(
+; CHECK-NEXT: %[[sub:.*]] = add i32 %A, %B
+; CHECK-NEXT: %[[and:.*]] = and i32 %[[sub]], 1
+; CHECK-NEXT: %[[cmp:.*]] = icmp ne i32 %[[and]], 0
+; CHECK-NEXT: ret i1 %[[cmp]]
+  %add = add i32 %A, -4
+  %sub = add nuw i32 %add, %B
+  %trunc = trunc i32 %sub to i1
+  ret i1 %trunc
+}