Clarify which type "Label" refers to (generic vs X86)

src/IceAssemblerMIPS32.h

Index: src/IceAssemblerMIPS32.h
diff --git a/src/IceAssemblerMIPS32.h b/src/IceAssemblerMIPS32.h
index 1503bf2f21e55ad425ba1dad958bfbc64464c09c..ca8eb04a7562a4ae2433524629d707cc32c61a0d 100644
--- a/src/IceAssemblerMIPS32.h
+++ b/src/IceAssemblerMIPS32.h
@@ -62,7 +62,7 @@ public:
     llvm::report_fatal_error("Not yet implemented.");
   }
 
-  Label *getOrCreateCfgNodeLabel(SizeT NodeNumber) override {
+  LabelBase *getCfgNodeLabel(SizeT NodeNumber) override {
     (void)NodeNumber;
     llvm_unreachable("Not yet implemented.");
   }