Fix heap poisoning

Was wrong register.

Test: test-art-host-run-test CC table lookup + poisoning.

Bug: 30162165
Change-Id: If347a40963bcc2fb9e21c54b5405c852bcd3af08
diff --git a/runtime/arch/x86_64/quick_entrypoints_x86_64.S b/runtime/arch/x86_64/quick_entrypoints_x86_64.S
index 8a4a334..32768b0 100644
--- a/runtime/arch/x86_64/quick_entrypoints_x86_64.S
+++ b/runtime/arch/x86_64/quick_entrypoints_x86_64.S
@@ -1098,7 +1098,7 @@
                                                                // Store the class pointer in the
                                                                // header.
                                                                // No fence needed for x86.
-    POISON_HEAP_REF ecx
+    POISON_HEAP_REF r8d
     movl %r8d, MIRROR_OBJECT_CLASS_OFFSET(%rax)
     movl %esi, MIRROR_ARRAY_LENGTH_OFFSET(%rax)
     ret                                                        // Fast path succeeded.