This flag is important for following optimizations. Therefore such `end_cow_mutation` instructions must not be removed.
Also, keep the flag in a SILCombine transformation.
rdar://119178823
Make filter APIs for UseList chainable by adding them to Sequence where Element == Operand
For example, it allows to write:
```
let singleUse = value.uses.ignoreDebugUses.ignoreUsers(ofType: EndAccessInst.self).singleUse
```
Also, add `UseList.getSingleUser(notOfType:)`
* begin_cow_mutation
* global_value
* strong_retain and strong_release
So far, those simplifications did only run in SILCombine. Now they are also considered in the swift Simplification pass.