[MLIR][DataFlow] Fix two crashes in DeadCodeAnalysis on empty/no-terminator regions (#188548)

Two related assertion failures in DeadCodeAnalysis when processing
OpenACC operations:

1. visitRegionBranchEdges (issue #187972): When a RegionSuccessor refers
to an empty region (no blocks), calling getSuccessor()->front()
dereferences a sentinel ilist iterator, crashing with
"\!NodePtr->isKnownSentinel()". Fix: skip successors whose region is
empty.

2. isRegionOrCallableReturn (issue #188408): When iterating over ops in
a nested acc region whose blocks do not have a required terminator,
Block::getTerminator() is called without first checking
mightHaveTerminator(), triggering "Assertion `mightHaveTerminator()'
failed". Fix: guard the getTerminator() call with mightHaveTerminator().

Fixes #187972, #188408

Assisted-by: Claude Code
GitOrigin-RevId: 820eaa427a14566856a3d5f8104605a4a352a822