[WinEH] Reset WinEHPrepare::SEHExceptionCodeSlot when we're done.

This caused a use-after-free on test/CodeGen/X86/win32-eh.ll
No functional change intended.

llvm-svn: 236561

llvm/lib/CodeGen/WinEHPrepare.cpp

@@ -934,6 +934,7 @@ bool WinEHPrepare::prepareExceptionHandlers(
   CleanupHandlerMap.clear();
   HandlerToParentFP.clear();
   DT = nullptr;
+  SEHExceptionCodeSlot = nullptr;
 
   return HandlersOutlined;
 }