This refines e7d521fb -- as we still
require clean up of the uint index register.

TEST=mjsunit/asm/float32array-negative-offset
R=joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com, bjaideep@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1919003004

Cr-Commit-Position: refs/heads/master@{#35778}