[mlir][spirv] Adapt subview legalization to the updated op semantics.

[mlir][spirv] Adapt subview legalization to the updated op semantics.

The subview semantics changes recently to allow for more natural
representation of constant offsets and strides. The legalization of
subview op for lowering to SPIR-V needs to account for this.
Also change the linearization to use the strides from the affine map
of a memref.

Differential Revision: https://reviews.llvm.org/D80270