vkd3d-shader: Begin validating the generated IR.

The validation code is meant both as a check that the frontend is behaving properly and as a sort of the documentation to establish what is allowed and what is not in the IR.

Currently an assertion is thrown if validation fails. I realize this is a rather strong proposal, but it's of course up for debate. In theory asserting here is the right thing, as it is expected that the frontend is generating correct IR code. However vkd3d is already used in production for many programs, and it could very well be that some of those are working properly even if the generated IR is somewhat out of specs; allowing the assertion might cause regressions as soon as enough checks are implemented in the validator. Please let me know your opinions. Solved in favor of a softer failure, and only when validation is enabled