[mlir][VectorOps] Introduce a `vector.fma` op that works on n-D vectors and...

[mlir][VectorOps] Introduce a `vector.fma` op that works on n-D vectors and lowers to `llvm.intrin.fmuladd`

Summary:
The `vector.fma` operation is portable enough across targets that we do not want
to keep it wrapped under `vector.outerproduct` and `llvm.intrin.fmuladd`.
This revision lifts the op into the vector dialect and implements the lowering to LLVM by using two patterns:
1. a pattern that lowers from n-D to (n-1)-D by unrolling when n > 2
2. a pattern that converts from 1-D to the proper LLVM representation

Reviewers: ftynse, stellaraccident, aartbik, dcaballe, jsetoain, tetuante

Reviewed By: aartbik

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74075