[LFTR] Rephrase getLoopTest into "based-on" check; NFCI

[LFTR] Rephrase getLoopTest into "based-on" check; NFCI

What we want to know here is whether we're already using this value
for the loop condition, so make the query about that. We can extend
this to a more general "based-on" relationship, rather than a direct
icmp use later.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@364715 91177308-0d34-0410-b5e6-96231b3b80d8