Avoid crash on some 64b ARM NEON platforms.
Avoid crash on some 64b ARM NEON platforms.

The compiler may choose to use x30 for a local loop counter;
ensure it's saved. Patch from kevin.petit@arm.com,
verified by benm@google.com.

R=djsollen@google.com

Review URL: https://codereview.chromium.org/786273003

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