[mlir][SCF] Improve `ForOp::getSuccessorRegions` (#177116)
- Loops with 0 iterations always branch back to the parent.
- Loops with 1 iteration always branch into the loop, then immediately
back to the parent.
This change improves the quality of data flow analyses (e.g., dead code
analysis). It is also in preparation of adding a generic region inlining
canonicalization pattern for `RegionBranchOpInterface` ops (#176641).
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Co-authored-by: Jakub Kuderski <jakub@nod-labs.com>
