Both for MergeStores and SuperWord, we have these similar tests:

test/hotspot/jtreg/compiler/c2/TestMergeStoresMemorySegment.java
test/hotspot/jtreg/compiler/loopopts/superword/TestMemorySegment.java

There are some examples that do not optimize.
For example the mixed mode for backing types does not optimize.
But also individual examples do not optimize as much as hoped for.
Especially for MergeStores, we have the issue that RangeChecks are not eliminated - i.e. not smeared.