[DirectX] Update DXILValueEnumerator for the new SwitchInst format (#174672)
Fixes #174485
This PR replicates the changes to
`llvm/lib/Bitcode/Writer/ValueEnumerator.cpp` from #170984 into
`llvm/lib/Target/DirectX/DXILWriter/DXILValueEnumerator.cpp` to support
the changes made to the SwitchInst format.
This PR also adds a new dxil-dis test to ensure that SwitchInst is being
successfully written to DXIL.