[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/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 831fb57..499e611 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -393,8 +393,7 @@
       return true;
 
     // Lifetime intrinsics are dead when their right-hand is undef.
-    if (II->getIntrinsicID() == Intrinsic::lifetime_start ||
-        II->getIntrinsicID() == Intrinsic::lifetime_end)
+    if (II->isLifetimeStartOrEnd())
       return isa<UndefValue>(II->getArgOperand(1));
 
     // Assumptions are dead if their condition is trivially true.  Guards on