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.

Author: eeckstein

include/swift/SIL/PrettyStackTrace.h

@@ -19,12 +19,12 @@
 #define SWIFT_SIL_PRETTYSTACKTRACE_H
 
 #include "swift/SIL/SILLocation.h"
+#include "swift/SIL/SILNode.h"
 #include "llvm/Support/PrettyStackTrace.h"
 
 namespace swift {
 class ASTContext;
 class SILFunction;
-class SILNode;
 
 void printSILLocationDescription(llvm::raw_ostream &out, SILLocation loc,
                                  ASTContext &ctx);
@@ -74,7 +74,7 @@ class PrettyStackTraceSILNode : public llvm::PrettyStackTraceEntry {
   const char *Action;
 
 public:
-  PrettyStackTraceSILNode(const char *action, const SILNode *node)
+  PrettyStackTraceSILNode(const char *action, SILNodePointer node)
     : Node(node), Action(action) {}
 
   virtual void print(llvm::raw_ostream &OS) const override;

include/swift/SIL/SILArgument.h

@@ -78,7 +78,7 @@ class SILArgument : public ValueBase {
   explicit SILArgument(ValueKind subClassKind, SILType type,
                        ValueOwnershipKind ownershipKind,
                        const ValueDecl *inputDecl = nullptr)
-      : ValueBase(subClassKind, type, IsRepresentative::Yes),
+      : ValueBase(subClassKind, type),
         parentBlock(nullptr), decl(inputDecl) {
     Bits.SILArgument.VOKind = static_cast<unsigned>(ownershipKind);
   }
@@ -106,7 +106,7 @@ class SILArgument : public ValueBase {
 
   static bool classof(const SILInstruction *) = delete;
   static bool classof(const SILUndef *) = delete;
-  static bool classof(const SILNode *node) {
+  static bool classof(SILNodePointer node) {
     return node->getKind() >= SILNodeKind::First_SILArgument &&
            node->getKind() <= SILNodeKind::Last_SILArgument;
   }
@@ -279,7 +279,7 @@ class SILPhiArgument : public SILArgument {
 
   static bool classof(const SILInstruction *) = delete;
   static bool classof(const SILUndef *) = delete;
-  static bool classof(const SILNode *node) {
+  static bool classof(SILNodePointer node) {
     return node->getKind() == SILNodeKind::SILPhiArgument;
   }
 };
@@ -322,7 +322,7 @@ class SILFunctionArgument : public SILArgument {
 
   static bool classof(const SILInstruction *) = delete;
   static bool classof(const SILUndef *) = delete;
-  static bool classof(const SILNode *node) {
+  static bool classof(SILNodePointer node) {
     return node->getKind() == SILNodeKind::SILFunctionArgument;
   }
 };