Restore CanType-based micro-optimizations.

This reverts commit 5036806e5a.
However, it preserves a pair of changes to the SIL optimizer
relating to walking through optional types.

include/swift/SIL/SILInstruction.h

@@ -4158,7 +4158,7 @@ public:
       exType = exMetatype.getInstanceType();
       concreteType = cast<MetatypeType>(concreteType).getInstanceType();
     }
-    assert(exType->isExistentialType());
+    assert(exType.isExistentialType());
     return concreteType;
   }