[MVT][SVE] Map between scalable vector IR Type and VTs

Adds a two way mapping between the scalable vector IR type and
corresponding SelectionDAG ValueTypes.

Reviewers: craig.topper, jeroen.dobbelaere, fhahn, rengolin, greened, rovka

Reviewed By: greened

Differential Revision: https://reviews.llvm.org/D47770

llvm-svn: 367832