[dfsan] Add a test case of storing zero

This covers a branch in storeShadow.

Reviewed-by: morehouse

Differential Revision: https://reviews.llvm.org/D92632

Author: stephan-yichao-zhao

llvm/test/Instrumentation/DataFlowSanitizer/store.ll

@@ -158,3 +158,9 @@ define void @store64(i64 %v, i64* %p) {
   store i64 %v, i64* %p
   ret void
 }
+
+define void @store_zero(i32* %p) {
+  ;  NO_COMBINE_PTR_LABEL: store i64 0, i64* {{.*}}, align 2
+  store i32 0, i32* %p
+  ret void
+}