[mlir] Affine symbols: do not expect AffineScope to always exist

In the affine symbol and dimension check, the code currently assumes
`getAffineScope` and its users `isValidDim` and `isValidSymbol` are only called
on values defined in regions that have a parent Op with `AffineScope` trait.
This is not necessarily the case, and these functions may be called on valid IR
that does not satisfy this assumption. Return `nullptr` from `getAffineScope`
if there is no parent op with `AffineScope` trait. Treat this case
conservatively in `isValidSymbol` by only accepting as symbols the values that
are guaranteed to be symbols (constants, and certain operations). No
modifications are necessary to `isValidDim` that delegates most of the work to
`isValidDim`.

Differential Revision: https://reviews.llvm.org/D81753

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