Fix a corner case in the new indvars promotion logic: if there

Fix a corner case in the new indvars promotion logic: if there
are multiple IV's in a loop, some of them may under go signed
or unsigned wrapping even if the IV that's used in the loop
exit condition doesn't. Restrict sign-extension-elimination
and zero-extension-elimination to only those that operate on
the original loop-controlling IV.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@64866 91177308-0d34-0410-b5e6-96231b3b80d8