[MLIR] Generalize/complete getEnclosingAffineForIfOps utility

Rename/generalize getEnclosingAffineForIfOps -> getEnclosingAffineOps.
The utility was originally written only for affine.for ops and then
extended for affine.if as well. It wasn't however updated when
affine.parallel was introduced -- in most cases, analysis has been used
in the presence of affine.for/if but not post parallelization. Extend
utility to also support affine.parallel ops; this allows future changes
to enable affine analysis and opts in the presence of affine.parallel
ops. Fix related stale comments.

This is NFC for all use cases in tree.

Change an associated assert to a utility failure.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D132326