Revert "x86/x86-64: Avoid temporary for read barrier field load."

Fault handler does not recognize the instruction
    F6 /0 ib    TEST r/m8, imm8
so we get crashes instead of NPEs.

Bug: 29966877
Bug: 12687968

This reverts commit ccf06d8f19a37432de4a3b768747090adfbd18ec.

Change-Id: Ib7db3b59f44c0d3ed5e24a20b6c6ee596a89d709
diff --git a/compiler/optimizing/code_generator_arm64.cc b/compiler/optimizing/code_generator_arm64.cc
index cc8985d..122c174 100644
--- a/compiler/optimizing/code_generator_arm64.cc
+++ b/compiler/optimizing/code_generator_arm64.cc
@@ -5041,7 +5041,7 @@
                     "art::mirror::CompressedReference<mirror::Object> and int32_t "
                     "have different sizes.");
 
-      // Slow path marking the GC root `root`.
+      // Slow path used to mark the GC root `root`.
       SlowPathCodeARM64* slow_path =
           new (GetGraph()->GetArena()) ReadBarrierMarkSlowPathARM64(instruction, root);
       codegen_->AddSlowPath(slow_path);
@@ -5239,7 +5239,7 @@
   // Object* ref = ref_addr->AsMirrorPtr()
   GetAssembler()->MaybeUnpoisonHeapReference(ref_reg);
 
-  // Slow path marking the object `ref` when it is gray.
+  // Slow path used to mark the object `ref` when it is gray.
   SlowPathCodeARM64* slow_path =
       new (GetGraph()->GetArena()) ReadBarrierMarkSlowPathARM64(instruction, ref);
   AddSlowPath(slow_path);