Subzero: Provide missing isMemoryWrite() method.

This is the result of an improper rebase in the previous CL.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1911983002 .

src/IceInst.h

diff --git a/src/IceInst.h b/src/IceInst.h
index f21aec8..352a5d4 100644
--- a/src/IceInst.h
+++ b/src/IceInst.h
@@ -979,6 +979,7 @@
     return Indexes[Pos];
   }
 
+  bool isMemoryWrite() const override { return false; }
   void dump(const Cfg *Func) const override;
   static bool classof(const Inst *Instr) {
     return Instr->getKind() == ShuffleVector;