SIL: let SingleValueInstruction only inherit from a single SILNode.

This removes the ambiguity when casting from a SingleValueInstruction to SILNode, which makes the code simpler. E.g. the "isRepresentativeSILNode" logic is not needed anymore.
Also, it reduces the size of the most used instruction class - SingleValueInstruction - by one pointer.

Conceptually, SILInstruction is still a SILNode. But implementation-wise SILNode is not a base class of SILInstruction anymore.
Only the two sub-classes of SILInstruction - SingleValueInstruction and NonSingleValueInstruction - inherit from SILNode. SingleValueInstruction's SILNode is embedded into a ValueBase and its relative offset in the class is the same as in NonSingleValueInstruction (see SILNodeOffsetChecker).
This makes it possible to cast from a SILInstruction to a SILNode without knowing which SILInstruction sub-class it is.
Casting to SILNode cannot be done implicitly, but only with an LLVM `cast` or with SILInstruction::asSILNode(). But this is a rare case anyway.

lib/SILOptimizer/Utils/LoadStoreOptUtils.cpp

@@ -120,7 +120,7 @@ void LSValue::reduceInner(LSLocation &Base, SILModule *M,
           Builder, RegularLocation::getAutoGeneratedLocation(),
           Base.getType(M, context).getObjectType(), Vals);
 
-  auto AvailVal = makeNewValueAvailable(AI.get(), InsertPt->getParentBlock(),
+  auto AvailVal = makeNewValueAvailable(AI.get(), InsertPt->getParent(),
                                         jointPostDomComputer);
 
   // This is the Value for the current base.