When rotating loops, put the original header at the bottom of the

When rotating loops, put the original header at the bottom of the
loop, making the resulting loop significantly less ugly.  Also, zap
its trivial PHI nodes, since it's easy.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@111255 91177308-0d34-0410-b5e6-96231b3b80d8