[mlir] [VectorOps] Progressive lowering of vector.broadcast

Summary:
Rather than having a full, recursive, lowering of vector.broadcast
to LLVM IR, it is much more elegant to have a progressive lowering
of each vector.broadcast into a lower dimensional vector.broadcast,
until only elementary vector operations remain. This results
in more elegant, step-wise code, that is easier to understand.
Also makes some optimizations in the generated code.

Reviewers: nicolasvasilache, mehdi_amini, andydavis1, grosul1

Reviewed By: nicolasvasilache

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78071