[MLIR][Affine] Add custom builders for AffineVectorLoadOp/AffineVectorStoreOp

Adding missing custom builders for AffineVectorLoadOp & AffineVectorStoreOp. In practice, it is difficult to correctly construct these ops without these builders (because the AffineMap is not included at construction time).

Differential Revision: https://reviews.llvm.org/D86380