[DirectX] Enable the Scalarizer pass in the DirectX backend

[farzonl]

We need to do a few things here:

• Add the legacy pass scalarizer interface back
• Hook it up to the DirectX backend
• Turn on the load store flag so loads and stores scalarize
• Update the non dx intrinsic test cases.

[davidcook-msft]

Will estimate after a final design is agreed upon.