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/func_translator.rs

@@ -135,7 +135,7 @@ fn declare_wasm_parameters<FE: FuncEnvironment + ?Sized>(
         let param_type = builder.func.signature.params[i];
         // There may be additional special-purpose parameters in addition to the normal WebAssembly
         // signature parameters. For example, a `vmctx` pointer.
-        if environ.is_wasm_parameter(&builder.func, i) {
+        if environ.is_wasm_parameter(&builder.func.signature, i) {
             // This is a normal WebAssembly signature parameter, so create a local for it.
             let local = Variable::new(next_local);
             builder.declare_var(local, param_type.value_type);