[mlir][memref] Make `LoadOp::verify` error more clear (#75831)

While debugging https://github.com/llvm/llvm-project/issues/71326, the
`LoadOp::verify` code and error were very confusing. This PR improves
that.

This code was a part from the reverted PR
https://github.com/llvm/llvm-project/pull/75519

. Fixing the
`-convert-vector-to-scf` issue is going to take a bit longer and this
code was out of scope anyway.

Co-authored-by: Benjamin Maxwell <macdue@dueutil.tech>