[MLIR][Linalg] Extend detensoring control flow model.

This patch extends the PureControlFlowDetectionModel to consider
detensoring br and cond_br operands.

See: https://github.com/google/iree/issues/1159#issuecomment-884322687,
for a disccusion on the need for such extension.

Reviewed By: silvas

Differential Revision: https://reviews.llvm.org/D107358