[InstCombine] Add additional test case for (add (zext (add nuw X, C2), C1). NFC

Add test where the zext has an additional use, but the entire
expression can be replaced with (zext X). Folding even though there
is an additional use would not increase the number of instructions.

Author: topperc

llvm/test/Transforms/InstCombine/add.ll

@@ -951,6 +951,21 @@ define i64 @test41(i32 %a) {
   ret i64 %sub
 }
 
+define i64 @test41_multiuse_constants_cancel(i32 %a) {
+; CHECK-LABEL: @test41_multiuse_constants_cancel(
+; CHECK-NEXT:    [[ADD:%.*]] = add nuw i32 [[A:%.*]], 1
+; CHECK-NEXT:    [[ZEXT:%.*]] = zext i32 [[ADD]] to i64
+; CHECK-NEXT:    [[REASS_ADD:%.*]] = shl nuw nsw i64 [[ZEXT]], 1
+; CHECK-NEXT:    [[EXTRAUSE:%.*]] = add nsw i64 [[REASS_ADD]], -1
+; CHECK-NEXT:    ret i64 [[EXTRAUSE]]
+;
+  %add = add nuw i32 %a, 1
+  %zext = zext i32 %add to i64
+  %sub = add i64 %zext, -1
+  %extrause = add i64 %zext, %sub
+  ret i64 %extrause
+}
+
 ; (add (zext (add nuw X, C2)), C) --> (zext (add nuw X, C2 + C))
 
 define <2 x i64> @test41vec(<2 x i32> %a) {