[mlir] Move getDimsOfType to StructuredOpsUtils.h.

Summary:
This change will bring all helpers that work with iterator types to one place.
Currently getDimsOfType is is declared in Linalg.h, but not directly included by
LinalgInterfaces. It worked so far only because all the places that include
LinalgInterfaces.h also include Linalg.h directly or indirectly.

Differential Revision: https://reviews.llvm.org/D134350