[NFC] Rename variable to workaround old gcc bug

Summary:
gcc 5.1 is still supported according to
https://releases.llvm.org/10.0.0/docs/GettingStarted.html

We're hitting the following bug due to a variable created in the loop header being the same as a variable used in the loop header:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54430

Reviewers: hctim, pcc

Subscribers: #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D81594