[mlir][spirv] Make MemRef memory space mapping pass more flexible

* Avoid restricting the pass to to builtin module ops. The pass
  should be able to run on any region ops.
* Avoid hardcoding func FuncOp when handling functions. Instead,
  use the function op interface.
* Assigns the default mapping in the constructor. So for cases
  where we are using the pass in a pipeline, we still have a
  meaningful default.

Along the way, dropped uncessary unrealized conversion casts and
use full conversion. The pass should be able to convert all sorts
of ops; there is really no need to have such bridages.

Reviewed By: kuhar

Differential Revision: https://reviews.llvm.org/D131409

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