[mlir] [VectorOps] Handle 'vector.shape_cast' lowering for all cases

Summary:
Even though this operation is intended for 1d/2d conversions currently,
leaving a semantic hole in the lowering prohibits proper testing of this
operation. This CL adds a straightforward reference implementation for the
missing cases.

Reviewers: nicolasvasilache, mehdi_amini, ftynse, reidtatge

Reviewed By: reidtatge

Subscribers: mehdi_amini, rriddle, jpienaar, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, stephenneuendorffer, Joonsoo, grosul1, frgossen, Kayjukh, jurahul, msifontes

Tags: #mlir

Differential Revision: https://reviews.llvm.org/D81503