[mlir][MemRefToLLVM] Remove the code for lowering subview

Subviews are supposed to be expanded before we hit the lowering
code.
The expansion is done with the pass called
expand-strided-metadata.

Add a test that demonstrate how these passes can be linked up to achieve
the desired lowering.

This patch is NFC in spirit but not in practice because `subview` gets
lowered into `reinterpret_cast(extract_strided_metadata, <some math>)`
which lowers in two memref descriptors (one for `reinterpert_cast` and
one for `extract_strided_metadata`), which creates some noise of the
form: `extractvalue(unrealized_cast(extractvalue[0]))[0]` that is
currently not simplified within MLIR but that is really just noop in
that case.

Differential Revision: https://reviews.llvm.org/D136377