[Feature Request] Support Native Vectors In Dxil

[pow2clk]

Per the dxil-vectors spec (pending), add support for native LLVM vectors in DXIL 1.9.

Is your feature request related to a problem? Please describe.
Native vectors are needed for long vectors which are useful for ML applications.

Describe the solution you'd like
DXIL inherits native vector support from LLVM IR, however, they were always disallowed by validation etc.

These will be allowed on any LLVM operators and elementwise DXIL intrinsics listed in the spec. In the initial implementation, some of these intrinsics may still be scalarized, but they will ultimately all take native vectors.

Execution tests are covered by #7068

Describe alternatives you've considered
A bespoke opaque vector type was considered for the broader solution, but using what existing LLVM vector support would allow fewer changes, less risk with tools based on LLVM, and consistency with future developments in modern LLVM.

[damyanp]

Figure out how we're going to track HLKs and any other aspects of this feature.

[pow2clk]

As I don't think this will involve more than one PR and I've filed an issue for execution tests, I've set this to planning again.