[mlir] Affine dim and symbol checkers: support detached regions

Functions checking whether an SSA value is a valid dimension or symbol for
affine operations can be called on values defined in a detached region (a
region that is not yet attached to an operation), for example, during parsing
or operation construction. These functions will attempt to uncondtionally
dereference a pointer to the parent operation of a region, which may be null
(as fixed by the previous commit, uninitialized before that). Since one cannot
know to which operation a region will be attached, conservatively this
operation would not be a valid affine scope and act accordingly, instead of
crashing.

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