[mlir] [VectorOps] Expose lowering pass options programmatically

The ConvertVectorToLLVM pass defines options that can be passed
on the command line (currently only reassociation of FP reductions
through -convert-vector-to-llvm='reassociate-fp-reductions). This
CL enables setting these options programmatically (forward looking
to more options than just reassociation, as well as setting the
values from code rather than command line).

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D83420