Register x25 is not reserved for anything else, so return it to the
Turbofan register allocator.

Change-Id: Ic905831683a825a1e1dd682552fd459dfc2323fe
Reviewed-on: https://chromium-review.googlesource.com/980976Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Cr-Commit-Position: refs/heads/master@{#52301}