LoopVectorizer: Don't count the induction variable multiple times

When estimating register pressure, don't count the induction variable mulitple
times. It is unlikely to be unrolled. This is currently disabled and hidden
behind a flag ("enable-ind-var-reg-heur").

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200371 91177308-0d34-0410-b5e6-96231b3b80d8