The 'key' value being passed into an array access should
be sign-extended on 64-bit platforms before being used to
index into memory.  Otherwise, garbage in the upper 32-bits
may result in a segmentation fault.

Minor fix to DoFlooringDivI to enforce 32-bit operations for calculation.

R=jyan@ca.ibm.com,michael_dawson@ca.ibm.com,mbrandy@us.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1822103002

Cr-Commit-Position: refs/heads/master@{#35000}