[mlir][VectorOps] Add ShapeCastOp to the vector ops dialect.

Summary:
Add ShapeCastOp to the vector ops dialect.

The shape_cast operation casts between an n-D source vector shape and a k-D result vector shape (the element type remains the same).

Reviewers: nicolasvasilache, aartbik

Reviewed By: nicolasvasilache

Subscribers: Joonsoo, merge_guards_bot, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, arpith-jacob, mgester, lucyrfox, liufengdb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73635