Adds the ExtensionType flag, which reflects the LoadExtType of a MaskedGatherSDNode.
This is set by the DAG combines added to fold a sext_inreg/and of a masked gather
into a [s|z]ext_masked_gather where legal.
Also updated SelectionDAGDumper::print_details so that details of the gather
load (is signed, is scaled & extension type) are printed.