[Tablegen] Avoid generating empty switch statements. NFC

This fixes an MSVC warning (warning C4065: switch statement contains 'default'
but no 'case' labels) introduced with revision 333293.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@333363 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed