Remove the sarg_t type and dummy_sarg_t instruction

They are no longer necessary with the new style backends

cranelift/codegen/src/ir/types.rs

@@ -400,7 +400,6 @@ impl Display for Type {
             f.write_str(match *self {
                 IFLAGS => "iflags",
                 FFLAGS => "fflags",
-                SARG_T => "sarg_t",
                 INVALID => panic!("INVALID encountered"),
                 _ => panic!("Unknown Type(0x{:x})", self.0),
             })