Now that we use an ilist of machine instructions, iterators are more robust

Now that we use an ilist of machine instructions, iterators are more robust
than before.  Because this is the case, we can compute the first non-phi
instruction once when de-phi'ing a block.  This shaves ~4s off of
phielimination of _Z7yyparsev in kimwitu++ from 109s -> 105s.  There are
still much more important gains to come.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13452 91177308-0d34-0410-b5e6-96231b3b80d8