[MLIR] Support for ReturnOps in memref map layout normalization

-- This commit handles the returnOp in memref map layout normalization.
-- An initial filter is applied on FuncOps which helps us know which functions can be
   a suitable candidate for memref normalization which doesn't lead to invalid IR.
-- Handles memref map normalization for external function assuming the external function
   is normalizable.

Differential Revision: https://reviews.llvm.org/D85226