CodeViewWidget: Get rid of magic values in OnInsertBLR() and OnInsertNOP()

A call like ReplaceAddress(address, 0) is pretty ambiguous; so is
ReplaceAddress(address, false), so use an enum class that tells people
straight-up what the replacer is.

This also gets rid of the really weird naming, where if 'blr' is true,
we'd be replacing the address with a NOP, rather than an actual BLR
instruction, so we invert that so it actually makes sense. There's no
actual bug fixed here though, considering the OnInsert functions
specified the correct values; it's literally just weird naming.

Source/Core/DolphinQt2/Debugger/CodeViewWidget.h

@@ -40,7 +40,13 @@ signals:
   void BreakpointsChanged();
 
 private:
-  void ReplaceAddress(u32 address, bool blr);
+  enum class ReplaceWith
+  {
+    BLR,
+    NOP
+  };
+
+  void ReplaceAddress(u32 address, ReplaceWith replace);
 
   void resizeEvent(QResizeEvent*) override;
   void keyPressEvent(QKeyEvent* event) override;