Update region slicing after llvm#167466

Author: ashermancinelli

mlir/python/mlir/dialects/scf.py

@@ -298,7 +298,7 @@ def default_block(self) -> Block:
 
     @builtins.property
     def case_regions(self) -> Sequence[Region]:
-        return [self.regions[1 + i] for i in range(len(self.cases))]
+        return self.regions[1:]
 
     def case_region(self, i: int) -> Region:
         return self.case_regions[i]