This PR adds support for shape casting from and to 0-D vectors.
Reviewed By: nicolasvasilache, hanchung, awarzynski
Differential Revision: https://reviews.llvm.org/D151851
Vector dialect patterns have grown enormously in the past year to a point where they are now impenetrable.
Start reorganizing them towards finer-grained control.
Differential Revision: https://reviews.llvm.org/D146736