[mlir] NFC: work around gcc-aarch64 v8.3 compilation issue in...

[mlir] NFC: work around gcc-aarch64 v8.3 compilation issue in getRegionBranchSuccessorOperands implementation.

https://reviews.llvm.org/rG25671db3d343 didn't quite do it because the underlying issue was that the specific compiler chokes on the second standard conversion sequence after the user-defined `MutableOperandRange::operator OperandRange() const` conversion (see https://en.cppreference.com/w/cpp/language/implicit_conversion).