[mlir] : Fix ViewOp shape folder for identity affine maps

Summary: Fix the ViewOpShapeFolder in case of no affine mapping associated with a Memref construct identity mapping.

Reviewers: nicolasvasilache

Subscribers: mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, arpith-jacob, mgester, lucyrfox, liufengdb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72735