Use the term "Function parameter" instead of "argument".

Rename the ArgumentType type to AbiParam since it describes the ABI
characteristics of a parameter or return value, not just the value type.

In Signature, rename members argument_types and return_types to "params"
and "returns". Again, they are not just types.

Fix a couple lingering references to "EBB arguments".

lib/cretonne/src/regalloc/coalescing.rs

@@ -418,14 +418,12 @@ impl<'a> Context<'a> {
                 self.func.dfg.display_inst(pred_inst, self.isa)
             );
 
-            // Never coalesce incoming function arguments on the stack. These arguments are
+            // Never coalesce incoming function parameters on the stack. These parameters are
             // pre-spilled, and the rest of the virtual register would be forced to spill to the
             // `incoming_arg` stack slot too.
             if let ValueDef::Param(def_ebb, def_num) = self.func.dfg.value_def(pred_val) {
                 if Some(def_ebb) == self.func.layout.entry_block() &&
-                    self.func.signature.argument_types[def_num]
-                        .location
-                        .is_stack()
+                    self.func.signature.params[def_num].location.is_stack()
                 {
                     dbg!("Isolating incoming stack parameter {}", pred_val);
                     let new_val = self.split_pred(pred_inst, pred_ebb, argnum, pred_val);