[mlir] [VectorOps] Use 'vector.flat_transpose' for 2-D 'vector.tranpose'

Summary:
Progressive lowering of vector.transpose into an operation that
is closer to an intrinsic, and thus the hardware ISA. Currently
under the common vector transform testing flag, as we prepare
deploying this transformation in the LLVM lowering pipeline.

Reviewers: nicolasvasilache, reidtatge, andydavis1, ftynse

Reviewed By: nicolasvasilache, ftynse

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

Tags: #llvm, #mlir

Differential Revision: https://reviews.llvm.org/D80772