Reassociate: Avoid iterator invalidation.

OpndPtrs stored pointers into the Opnd vector that became invalid when the
vector grows. Store indices instead. Sadly I only have a large testcase that
only triggers under valgrind, so I didn't include it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178793 91177308-0d34-0410-b5e6-96231b3b80d8