[IR] Add Instruction::isLifetimeStartOrEnd, NFC

Instruction::isLifetimeStartOrEnd() checks whether an Instruction is an
llvm.lifetime.start or an llvm.lifetime.end intrinsic.

This was suggested as a cleanup in D55967.

Differential Revision: https://reviews.llvm.org/D56019

llvm-svn: 349964
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index dbaa277..efbab47 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -3822,8 +3822,7 @@
     const IntrinsicInst *II = dyn_cast<IntrinsicInst>(U);
     if (!II) return false;
 
-    if (II->getIntrinsicID() != Intrinsic::lifetime_start &&
-        II->getIntrinsicID() != Intrinsic::lifetime_end)
+    if (!II->isLifetimeStartOrEnd())
       return false;
   }
   return true;