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/spec.rs

@@ -140,7 +140,9 @@ pub trait TargetEnvironment {
 pub trait FuncEnvironment: TargetEnvironment {
     /// Is the given parameter of the given function a wasm-level parameter, as opposed to a hidden
     /// parameter added for use by the implementation?
-    fn is_wasm_parameter(&self, func: &ir::Function, index: usize) -> bool;
+    fn is_wasm_parameter(&self, signature: &ir::Signature, index: usize) -> bool {
+        signature.params[index].purpose == ir::ArgumentPurpose::Normal
+    }
 
     /// Should the code be structured to use a single `fallthrough_return` instruction at the end
     /// of the function body, rather than `return` instructions as needed? This is used by VMs