Add missing vtable anchor's.

The following description is from http://reviews.llvm.org/D15481:

ICmpInst, GetElementPtrInst and PHINode have no anchor functions. This causes the vtable and the type info (if RTTI is enabled in user code) to be emitted in multiple translation units.

Before 3.7, the destructors were the key functions for these nodes, but they have been removed.

There have been discussions about this here: http://lists.llvm.org/pipermail/llvm-dev/2015-August/089010.html and here: http://lists.llvm.org/pipermail/llvm-dev/2015-December/092921.html.

Patch by Visoiu Mistrih Francis

llvm-svn: 255538

llvm/include/llvm/IR/Instructions.h

@@ -839,6 +839,8 @@ class GetElementPtrInst : public Instruction {
   Type *SourceElementType;
   Type *ResultElementType;
 
+  void anchor() override;
+
   GetElementPtrInst(const GetElementPtrInst &GEPI);
   void init(Value *Ptr, ArrayRef<Value *> IdxList, const Twine &NameStr);
 
@@ -1097,6 +1099,8 @@ DEFINE_TRANSPARENT_OPERAND_ACCESSORS(GetElementPtrInst, Value)
 /// must be identical types.
 /// \brief Represent an integer comparison operator.
 class ICmpInst: public CmpInst {
+  void anchor() override;
+
   void AssertOK() {
     assert(getPredicate() >= CmpInst::FIRST_ICMP_PREDICATE &&
            getPredicate() <= CmpInst::LAST_ICMP_PREDICATE &&
@@ -2426,6 +2430,8 @@ DEFINE_TRANSPARENT_OPERAND_ACCESSORS(InsertValueInst, Value)
 // scientist's overactive imagination.
 //
 class PHINode : public Instruction {
+  void anchor() override;
+
   void *operator new(size_t, unsigned) = delete;
   /// ReservedSpace - The number of operands actually allocated.  NumOperands is
   /// the number actually in use.

llvm/lib/IR/Instructions.cpp

@@ -87,6 +87,8 @@ const char *SelectInst::areInvalidOperands(Value *Op0, Value *Op1, Value *Op2) {
 //                               PHINode Class
 //===----------------------------------------------------------------------===//
 
+void PHINode::anchor() {}
+
 PHINode::PHINode(const PHINode &PN)
     : Instruction(PN.getType(), Instruction::PHI, nullptr, PN.getNumOperands()),
       ReservedSpace(PN.getNumOperands()) {
@@ -1503,6 +1505,8 @@ FenceInst::FenceInst(LLVMContext &C, AtomicOrdering Ordering,
 //                       GetElementPtrInst Implementation
 //===----------------------------------------------------------------------===//
 
+void GetElementPtrInst::anchor() {}
+
 void GetElementPtrInst::init(Value *Ptr, ArrayRef<Value *> IdxList,
                              const Twine &Name) {
   assert(getNumOperands() == 1 + IdxList.size() &&
@@ -3389,6 +3393,8 @@ CmpInst::Predicate CmpInst::getInversePredicate(Predicate pred) {
   }
 }
 
+void ICmpInst::anchor() {}
+
 ICmpInst::Predicate ICmpInst::getSignedPredicate(Predicate pred) {
   switch (pred) {
     default: llvm_unreachable("Unknown icmp predicate!");