[IR] fix declaration of shuffle mask

An address sanitizer bot flagged this as a potential bug.

llvm-svn: 341084
diff --git a/llvm/lib/IR/Instructions.cpp b/llvm/lib/IR/Instructions.cpp
index 2bf9f0b..266345b 100644
--- a/llvm/lib/IR/Instructions.cpp
+++ b/llvm/lib/IR/Instructions.cpp
@@ -1780,7 +1780,7 @@
     return false;
 
   // The first part of the mask must choose elements from exactly 1 source op.
-  ArrayRef<int> Mask = getShuffleMask();
+  SmallVector<int, 16> Mask = getShuffleMask();
   if (!isIdentityMaskImpl(Mask, NumOpElts))
     return false;