don't bother every time going around the main worklist. This speeds up a
release-asserts opt -std-compile-opts on 403.gcc by about 4% (1.5s). It
seems to speed up the most expensive instances of instcombine by ~10%.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@84171 91177308-0d34-0410-b5e6-96231b3b80d8