[Reassociate] Use a range loop for clarity. NFCI.

While here, rename `i` to `Rank` as the latter is more
self-explanatory (and this code also uses `I` two lines below to
identify an Instruction).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310238 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed