[liftoff] use LoadFullPointer for root slots

Root slots are full pointer size. Using tagged ptr loads
will lead to errors on big endian platform.

Change-Id: I477597fadee025b6fb060d67a83bee60290aeb14
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2857636
Commit-Queue: Junliang Yan <junyan@redhat.com>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74273}

src/wasm/baseline/liftoff-compiler.cc

@@ -5818,9 +5818,8 @@ class LiftoffCompiler {
 
   void LoadNullValue(Register null, LiftoffRegList pinned) {
     LOAD_INSTANCE_FIELD(null, IsolateRoot, kSystemPointerSize, pinned);
-    __ LoadTaggedPointer(null, null, no_reg,
-                         IsolateData::root_slot_offset(RootIndex::kNullValue),
-                         pinned);
+    __ LoadFullPointer(null, null,
+                       IsolateData::root_slot_offset(RootIndex::kNullValue));
   }
 
   void LoadExceptionSymbol(Register dst, LiftoffRegList pinned,