Fixed issue with UseScratchRegisterScope that made test fail on r1
and wrong register usage on all arch variants.

Bug: 
Change-Id: Id89ff84046d012dd0767b9031b2719f9a95a08b8
Reviewed-on: https://chromium-review.googlesource.com/667139Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@imgtec.com>
Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@imgtec.com>
Cr-Commit-Position: refs/heads/master@{#48030}