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/wasm/src/environ/dummy.rs

@@ -201,10 +201,6 @@ impl<'dummy_environment> TargetEnvironment for DummyFuncEnvironment<'dummy_envir
 }
 
 impl<'dummy_environment> FuncEnvironment for DummyFuncEnvironment<'dummy_environment> {
-    fn is_wasm_parameter(&self, func: &ir::Function, index: usize) -> bool {
-        func.signature.params[index].purpose == ir::ArgumentPurpose::Normal
-    }
-
     fn return_mode(&self) -> ReturnMode {
         self.return_mode
     }