Fix PR16508.

When phis get lowered, destination copies are inserted using an iterator that is
determined once for all phis in the block, which BuildMI interprets as a request
to insert an instruction directly before the iterator. In the case of a cyclic
phi, source copies may also be inserted directly before this iterator, which can
cause source copies to be inserted before destination copies. The fix is to keep
an iterator to the last phi and then advance it while lowering each phi in order
to insert destination copies directly after the phis.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185363 91177308-0d34-0410-b5e6-96231b3b80d8