Added MD_invariant_group to LLVMContext

[reames]

LGTM

[Prazek]

When LGTMing, please accept revision, so I can close it

include/llvm/IR/LLVMContext.h

enum {
     MD_alias_scope = 7, // "alias.scope"
     MD_noalias = 8, // "noalias",
     MD_nontemporal = 9, // "nontemporal"
     MD_mem_parallel_loop_access = 10, // "llvm.mem.parallel_loop_access"
     MD_nonnull = 11, // "nonnull"
     MD_dereferenceable = 12, // "dereferenceable"
     MD_dereferenceable_or_null = 13, // "dereferenceable_or_null"
     MD_make_implicit = 14, // "make.implicit"
-    MD_unpredictable = 15 // "unpredictable"
+    MD_unpredictable = 15, // "unpredictable"
+    MD_invariant_group = 16 // "invariant.group"
   };
 
   /// getMDKindID - Return a unique non-zero ID for the specified metadata kind.
   /// This ID is uniqued across modules in the current LLVMContext.
   unsigned getMDKindID(StringRef Name) const;
 
   /// getMDKindNames - Populate client supplied SmallVector with the name for
   /// custom metadata IDs registered in this LLVMContext.

lib/CodeGen/CodeGenPrepare.cpp

for (auto &BB : F) {
 
     DEBUG(dbgs() << "After branch condition splitting\n"; BB.dump();
           TmpBB->dump());
   }
   return MadeChange;
 }
 
 void CodeGenPrepare::stripInvariantGroupMetadata(Instruction &I) {
-  if (auto *InvariantMD = I.getMetadata("invariant.group"))
+  if (auto *InvariantMD = I.getMetadata(LLVMContext::MD_invariant_group))
     I.dropUnknownNonDebugMetadata(InvariantMD->getMetadataID());
 }

lib/IR/LLVMContext.cpp

assert(MakeImplicitID == MD_make_implicit &&
          "make.implicit kind id drifted");
   (void)MakeImplicitID;
 
   // Create the 'unpredictable' metadata kind.
   unsigned UnpredictableID = getMDKindID("unpredictable");
   assert(UnpredictableID == MD_unpredictable &&
          "unpredictable kind id drifted");
   (void)UnpredictableID;
+
+  // Create the 'invariant.group' metadata kind.
+  unsigned InvariantGroupId = getMDKindID("invariant.group");
+  assert(InvariantGroupId == MD_invariant_group &&
+         "invariant.group kind id drifted");
+  (void)InvariantGroupId;
+
 }
 LLVMContext::~LLVMContext() { delete pImpl; }
 
 void LLVMContext::addModule(Module *M) {
   pImpl->OwnedModules.insert(M);
 }
 
 void LLVMContext::removeModule(Module *M) {