Add explanatory comment.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@149416 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/ARM/ARMConstantIslandPass.cpp b/lib/Target/ARM/ARMConstantIslandPass.cpp
index e6f49ab..1828cbb 100644
--- a/lib/Target/ARM/ARMConstantIslandPass.cpp
+++ b/lib/Target/ARM/ARMConstantIslandPass.cpp
@@ -834,6 +834,7 @@
     if (&*I == MI) return Offset;
     Offset += TII->GetInstSizeInBytes(I);
   }
+  // Unreachable.
 }
 
 /// CompareMBBNumbers - Little predicate function to sort the WaterList by MBB