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.

include/swift/SILOptimizer/Utils/LoadStoreOptUtils.h

@@ -269,7 +269,7 @@ public:
     }
     auto Res = Path.getValue().createExtract(Val, &*InsertPt, true);
     if (Val != Base) {
-      Res = makeCopiedValueAvailable(Res, Inst->getParentBlock(),
+      Res = makeCopiedValueAvailable(Res, Inst->getParent(),
                                      jointPostDomComputer);
       Builder.emitEndBorrowOperation(InsertPt->getLoc(), Val);
       // Insert a destroy on the Base