This fixes#126784 for the DirectX backend.
This bug was marked critical for DX so it is going to go in first. At
least one register class needs to be added via `addRegisterClass` for
`RegClassForVT` to be valid.
Further for costing information used by loop unroll and other
optimizations to be valid we need to call `computeRegisterProperties`.
This change does both of these.
The test cases confirm that we can fetch costing information off of
`getRegisterInfo` and that `DirectXTargetLowering` maps `i32` typed
registers to `DXILClassRegClass`.
dc764f5c68