[wasm] Remove temp_instance in LoadTableSegments

The temp_instance is not used but had an enormous performance impact.
The instantiation time of the zengarden WebAssembly module improved now
from 12 seconds to less than a second.

[email protected]
BUG=v8:6146

Change-Id: Ibac7809b3b6092032abd77ed94f639c09f62d816
Reviewed-on: https://chromium-review.googlesource.com/458918
Commit-Queue: Andreas Haas <[email protected]>
Reviewed-by: Clemens Hammacher <[email protected]>
Cr-Commit-Position: refs/heads/master@{#44114}

src/wasm/wasm-module.cc

@@ -2235,11 +2235,6 @@ class InstantiationHelper {
               // TODO(titzer): We compile JS->WASM wrappers for functions are
               // not exported but are in an exported table. This should be done
               // at module compile time and cached instead.
-              WasmInstance temp_instance(module_);
-              temp_instance.context = isolate_->native_context();
-              temp_instance.mem_size = 0;
-              temp_instance.mem_start = nullptr;
-              temp_instance.globals_start = nullptr;
 
               Handle<Code> wrapper_code =
                   js_to_wasm_cache_.CloneOrCompileJSToWasmWrapper(