UnimplementedLoweringError's message now includes the instruction name.

src/IceInst.h

Index: src/IceInst.h
diff --git a/src/IceInst.h b/src/IceInst.h
index 85fde52b316fdb67d4f23481a277f92997673838..c1d5bcccff0f2022b48a8115947d0dc7f14f3da8 100644
--- a/src/IceInst.h
+++ b/src/IceInst.h
@@ -76,6 +76,7 @@ public:
   };
   static_assert(Target <= Target_Max, "Must not be above max.");
   InstKind getKind() const { return Kind; }
+  virtual IceString getInstName() const;
 
   InstNumberT getNumber() const { return Number; }
   void renumber(Cfg *Func);
@@ -288,6 +289,9 @@ public:
         InstArithmetic(Func, Op, Dest, Source1, Source2);
   }
   OpKind getOp() const { return Op; }
+
+  virtual IceString getInstName() const override;
+
   static const char *getOpName(OpKind Op);
   bool isCommutative() const;
   void dump(const Cfg *Func) const override;