[mlir][bufferization] Add support for non-unique `func.return` (#114017)

Multiple `func.return` ops inside of a `func.func` op are now supported
during bufferization. This PR extends the code base in 3 places:

- When inferring function return types, `memref.cast` ops are folded
away only if all `func.return` ops have matching buffer types. (E.g., we
don't fold if two `return` ops have operands with different layout
maps.)
- The alias sets of all `func.return` ops are merged. That's because
aliasing is a "may be" property.
- The equivalence sets of all `func.return` ops are taken only if they
match. If different `func.return` ops have different equivalence sets
for their operands, the equivalence information is dropped. That's
because equivalence is a "must be" property.

This commit is in preparation of removing the deprecated
`func-bufferize` pass. That pass can bufferize functions with multiple
`return` ops.

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