Add C++ shape inference for SVD.

This also adds Min(), Max(), and Subtract() operators and a few convenience methods to the InferenceContext. Change test utils to emit a human readable error message in case the user forgot to set the inference function.

Refactored shape_inference* a bit to enforce the invariant that a Dimension or DimensionOrConstant is always non-negative or equal to InferenceContext::kUnknownDim.
This made it possible to tighten & simplify the arithmetic operations a bit.
Change: 129385995