Fix index register assignment in LoadFieldByIndex for arm, arm64, and mips.

This instruciton clobbers the index register.

BUG=368243
LOG=N
TEST=mjsunit/regress/regress-368243
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/269273003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21196 ce2b1a6d-e550-0410-aec6-3dcde31c8c00