This refines 6382a610 -- as we still
require clean up of the uint index register.

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

Conflicts:
	src/compiler/s390/code-generator-s390.cc

Review-Url: https://codereview.chromium.org/1921073003
Cr-Commit-Position: refs/heads/master@{#35879}