[mlir][vector] Separate out vector transfer + tensor slice patterns

These patterns touches the structure generated from tiling so it
affects later steps like bufferization and vector hoisting.
Instead of putting them in canonicalization, this commit creates
separate entry points for them to be called explicitly.

This is NFC regarding the functionality and tests of those patterns.
It also addresses two TODO items in the codebase.

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D150702