[CIR] Add RegionBranchOpInterface unit tests and fix control flow bugs (#195433)
Add unit tests for RegionBranchOpInterface implementations across CIR
control flow operations: IfOp, ScopeOp, TernaryOp, SwitchOp, WhileOp,
ForOp, DoWhileOp, and TryOp. The tests verify successor regions,
terminator successors, loop detection, repetitive region marking, and
op/terminator successor consistency.
Fix a missing return in ConditionOp::getSuccessorRegions that caused
fallthrough from the loop case to an unconditional cast<AwaitOp>,
crashing when the parent is a loop operation.
Fix IfOp::getSuccessorRegions to report parent exit as a successor
when the else region is absent, correctly modeling the case where the
condition is false.