[mlir][DeadCodeAnalysis] Don't Require `RegionBranchTerminatorOpInterface` in...

[mlir][DeadCodeAnalysis] Don't Require `RegionBranchTerminatorOpInterface` in `visitRegionTerminator()` (#69043)

Fix for a crash reported in #64975. 

The crash occurs in the cast located
[here](https://github.com/llvm/llvm-project/blob/ece5dd101c7e4dc2fd23428abd312f75fd3d3eaf/mlir/lib/Analysis/DataFlow/DeadCodeAnalysis.cpp#L262)
because `llvm.unreachable` doesn't implement
`RegionBranchTerminatorOpInterface`.

The crash is caused by `DeadCodeAnalysis` assuming that
`isa<RegionBranchOpInterface>(op->getParentOp())` implies
`isa<RegionBranchTerminatorOpInterface>(op)` in
`DeadCodeAnalysis::visit()`.

This patch tried to fix this by enabling the analysis to proceed
regardless of whether `op` is a `RegionBranchTerminatorOpInterface`.