[TableGen] Avoid generating switch with just default

Summary:
Switch with just default causes an MSVC warning (warning C4065: switch statement
contains 'default' but no 'case' labels).

Change-Id: I9ddeccdef93666256b5454b164b567b73b488461

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81021