[mlir][NFC] Rename `MemRefType::getMemorySpace` to `getMemorySpaceAsInt`

Just a pure method renaming.

It is a preparation step for replacing "memory space as raw integer"
with more generic "memory space as attribute", which will be done in
separate commit.

The `MemRefType::getMemorySpace` method will return `Attribute` and
become the main API, while `getMemorySpaceAsInt` will be declared as
deprecated and will be replaced in all in-tree dialects (also in separate
commits).

Reviewed By: mehdi_amini, rriddle

Differential Revision: https://reviews.llvm.org/D97476