Don't split up stores into an existential payload in DI

Predictable memory optimization would do the wrong thing upon
encountering a copy_addr into the payload of an existential --
if the source of the copy was otherwise promotable, it would
attempt to explode the copy_addr into a sequence of scalars.

The 'sub-element' counting code assumed that the source and
destination type had the same shape when tuples and structs
were exploded, so if the existential payload was a struct or
tuple with multiple elements, invariants would be lost in
the bookkeeping code.

So it appears that if the destination of a store is an
existential payload it does not really make sense to consider
the uses individually in DI.

I'm not aware of any code that SILGen currently emits which
may trigger this, but it came up while I was testing some
other stuff.

Author: slavapestov

lib/SILPasses/EarlySIL/DIMemoryUseCollector.cpp

@@ -749,22 +749,16 @@ void ElementUseCollector::collectUses(SILValue Pointer, unsigned BaseEltNo) {
       continue;
     }
 
-    // init_existential_addr is modeled as an initialization store, where the
-    // uses are treated as subelement accesses.
+    // init_existential_addr is modeled as an initialization store.
     if (isa<InitExistentialAddrInst>(User)) {
       assert(!InStructSubElement &&
              "init_existential_addr should not apply to struct subelements");
       Uses.push_back(DIMemoryUse(User, DIUseKind::Initialization,
                                  BaseEltNo, 1));
-
-      // Set the "InEnumSubElement" flag (so we don't consider tuple indexes to
-      // index across elements) and recursively process the uses.
-      llvm::SaveAndRestore<bool> X(InEnumSubElement, true);
-      collectUses(SILValue(User, 0), BaseEltNo);
       continue;
     }
 
-    // inject_enum_addr is treated as a store unconditionally.
+    // inject_enum_addr is modeled as an initialization store.
     if (isa<InjectEnumAddrInst>(User)) {
       assert(!InStructSubElement &&
              "inject_enum_addr the subelement of a struct unless in a ctor");