[wasm][cleanup] Use macros to load instance fields

Use the existing macros to load MemoryStart and MemorySize from the
instance.

R=titzer@chromium.org

Change-Id: Iaa597fedcfc4581503d7cdf2fb94da6e285cc545
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1538122Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60434}

src/compiler/wasm-compiler.cc

@@ -3040,16 +3040,12 @@ void WasmGraphBuilder::InitInstanceCache(
   DCHECK_NOT_NULL(instance_node_);
 
   // Load the memory start.
-  instance_cache->mem_start = SetEffect(graph()->NewNode(
-      mcgraph()->machine()->Load(MachineType::UintPtr()), instance_node_.get(),
-      mcgraph()->Int32Constant(WASM_INSTANCE_OBJECT_OFFSET(MemoryStart)),
-      Effect(), Control()));
+  instance_cache->mem_start =
+      LOAD_INSTANCE_FIELD(MemoryStart, MachineType::UintPtr());
 
   // Load the memory size.
-  instance_cache->mem_size = SetEffect(graph()->NewNode(
-      mcgraph()->machine()->Load(MachineType::UintPtr()), instance_node_.get(),
-      mcgraph()->Int32Constant(WASM_INSTANCE_OBJECT_OFFSET(MemorySize)),
-      Effect(), Control()));
+  instance_cache->mem_size =
+      LOAD_INSTANCE_FIELD(MemorySize, MachineType::UintPtr());
 
   if (untrusted_code_mitigations_) {
     // Load the memory mask.