[mlir] [VectorOps] Add 'vector.flat_transpose' operation

Summary:
Provides a representation of the linearized LLVM instrinsic.
With tests and lowering implementation to LLVM IR dialect.
Prepares better lowering for 2-D vector.transpose.

Reviewers: nicolasvasilache, ftynse, reidtatge, bkramer, dcaballe

Reviewed By: ftynse, dcaballe

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80419