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.

llvm-svn: 13452