InitializeStaticGlobals: fix a SIL verifier crash
When merging stores in a global initializer, it's possible that the merged store is inserted at the wrong location, causing a SIL verifier error.
This is hard to reproduce, but can happen.
The merged store must be inserted _after_ all other stores. Instead it's inserted after the store of the last property. Now, if properties are _not_ initialized in the order they are declared, this problem can show up.
rdar://117189962
