NFC. Clean up memref utils library

NFC. Clean up memref utils library. This library had a single function
that was completely misplaced. MemRefUtils is expected to be (also per
its comment) a library providing analysis/transforms utilities on memref
dialect ops or memref types. However, in reality it had a helper that
was depended upon by the MemRef dialect, i.e., it was a helper for the
dialect ops library and couldn't contain anything that itself depends on
the MemRef dialect. Move the single method to the memref dialect that
will now allow actual utilities depending on the memref dialect to be
placed in it.

Put findDealloc in the `memref` namespace. This is a pure move.

Differential Revision: https://reviews.llvm.org/D121273