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/SIL/IR/SILFunction.cpp

@@ -246,10 +246,10 @@ void SILFunction::numberValues(llvm::DenseMap<const SILNode*, unsigned> &
     for (auto &I : BB) {
       auto results = I.getResults();
       if (results.empty()) {
-        ValueToNumberMap[&I] = idx++;
+        ValueToNumberMap[I.asSILNode()] = idx++;
       } else {
         // Assign the instruction node the first result ID.
-        ValueToNumberMap[&I] = idx;
+        ValueToNumberMap[I.asSILNode()] = idx;
         for (auto result : results) {
           ValueToNumberMap[result] = idx++;
         }