only try to fold constantexpr operands when the worklist is first populated,

only try to fold constantexpr operands when the worklist is first populated,
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