[RS4GC] Clean up `find_index`; NFC

 - Bring it up to the LLVM Coding Style
 - Sink it inside `CreateGCRelocates`, which is its only user

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@250785 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed