[wasm] Make wasm-compiler.cc independent of JSGraph.

This removes the last dependencies on {JSGraph} from the wasm compiler.
Since the pipeline does not run appropriate lowerings, access to the
simplified or the javascript operator level is disallowed. With this
change that rule is statically enforced.

R=thibaudm@chromium.org

Change-Id: I4a97e63ffc5df1b7704477ac5c68707f5b195166
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1758305Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63234}

src/compiler/wasm-compiler.h

@@ -272,7 +272,6 @@ class WasmGraphBuilder {
     Node* arr[] = {fst, more...};
     return Return(ArrayVector(arr));
   }
-  Node* ReturnVoid();
   Node* Unreachable(wasm::WasmCodePosition position);
 
   Node* CallDirect(uint32_t index, Node** args, Node*** rets,