Add a is_wasm_parameter method to the wasm FuncEnvironment. (#1329)

This provides a more flexible way to allow embedding to tell
cranelift-wasm which function parameters are hidden, and which should be
translated as wasm user variables.

This replaces https://github.com/bytecodealliance/cranelift/pull/1086.

cranelift/wasm/src/environ/dummy.rs

@@ -201,6 +201,10 @@ 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
     }