[mlir][linalg] lower index operations during linalg to vector lowering.

[mlir][linalg] lower index operations during linalg to vector lowering.

The patch extends the vectorization pass to lower linalg index operations to vector code. It allocates constant 1d vectors that enumerate the indexes along the iteration dimensions and broadcasts/transposes these 1d vectors to the iteration space.

Differential Revision: https://reviews.llvm.org/D100373