[MLIR] Helper class referencing MemRefType to unify runner implementations.

Summary:
Add DynamicMemRefType which can reference one of the statically ranked StridedMemRefType or a UnrankedMemRefType so that runner utils only need to be implemented once.

There is definitely room for more clean up and unification, but I will keep that for follow-ups.

Reviewers: nicolasvasilache

Reviewed By: nicolasvasilache

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80513