First allocatable register must be same as the argument
count register.
This change fixes problems with optimized tests.

Bug: 
Change-Id: I06731c7f27c5f6a761ebdeb3c3c7b2a6c14d7c1d
Reviewed-on: https://chromium-review.googlesource.com/716224Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@imgtec.com>
Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@imgtec.com>
Cr-Commit-Position: refs/heads/master@{#48540}