Correctly count the number of wasm parameters. (#1337)

* Correctly count the number of wasm parameters.

Following up on #1329, this further replaces `num_normal_params` with a function
which calls `is_wasm_parameter` to correctly count the number of wasm
parameters a function has.

* Move is_wasm_parameter's implementation into the trait.

cranelift/codegen/src/ir/extfunc.rs

@@ -88,16 +88,6 @@ impl Signature {
             .count()
     }
 
-    /// Count the number of normal parameters in a signature.
-    /// Exclude special-purpose parameters that represent runtime stuff and not WebAssembly
-    /// arguments.
-    pub fn num_normal_params(&self) -> usize {
-        self.params
-            .iter()
-            .filter(|arg| arg.purpose == ArgumentPurpose::Normal)
-            .count()
-    }
-
     /// Does this signature take an struct return pointer parameter?
     pub fn uses_struct_return_param(&self) -> bool {
         self.uses_special_param(ArgumentPurpose::StructReturn)