IR: Reserve an MDKind for !llvm.loop; NFC

This reserves an MDKind for !llvm.loop, which allows callers to avoid a
string-based lookup.  I'm not sure why it was missing.

There should be no functionality change here, just a small compile-time
speedup.

llvm-svn: 264371
diff --git a/llvm/lib/IR/LLVMContext.cpp b/llvm/lib/IR/LLVMContext.cpp
index 77cefd0..151c205 100644
--- a/llvm/lib/IR/LLVMContext.cpp
+++ b/llvm/lib/IR/LLVMContext.cpp
@@ -128,6 +128,11 @@
   assert(AlignID == MD_align && "align kind id drifted");
   (void)AlignID;
 
+  // Create the 'llvm.loop' metadata kind.
+  unsigned LoopID = getMDKindID("llvm.loop");
+  assert(LoopID == MD_loop && "llvm.loop kind id drifted");
+  (void)LoopID;
+
   auto *DeoptEntry = pImpl->getOrInsertBundleTag("deopt");
   assert(DeoptEntry->second == LLVMContext::OB_deopt &&
          "deopt operand bundle id drifted!");