Handle ABI arguments correctly in the reload pass.

Values passed as arguments to calls and return instructions may also be
reload candidates.

lib/cretonne/src/ir/valueloc.rs

@@ -104,6 +104,14 @@ impl ArgumentLoc {
         }
     }
 
+    /// Is this a register location?
+    pub fn is_reg(&self) -> bool {
+        match self {
+            &ArgumentLoc::Reg(_) => true,
+            _ => false,
+        }
+    }
+
     /// Return an object that can display this argument location, using the register info from the
     /// target ISA.
     pub fn display<'a, R: Into<Option<&'a RegInfo>>>(self, regs: R) -> DisplayArgumentLoc<'a> {