[mips] Fix broken build after 9302db48.

Change-Id: Ic156eefa7ee2de568a368c44eca0ba80ab8828b0
Reviewed-on: https://chromium-review.googlesource.com/c/1388567Reviewed-by: Sreten Kovacevic <skovacevic@wavecomp.com>
Commit-Queue: Sreten Kovacevic <skovacevic@wavecomp.com>
Cr-Commit-Position: refs/heads/master@{#58462}

src/mips/assembler-mips-inl.h

@@ -178,7 +178,7 @@ void Assembler::deserialization_set_target_internal_reference_at(
   }
 }
 
-HeapObject* RelocInfo::target_object() {
+HeapObject RelocInfo::target_object() {
   DCHECK(IsCodeTarget(rmode_) || IsEmbeddedObject(rmode_));
   return HeapObject::cast(
       ObjectPtr(Assembler::target_address_at(pc_, constant_pool_)));