R600: avoid calling std::next on an iterator that might be end()

This was causing my llc to go into an infinite loop on
CodeGen/R600/address-space.ll (just triggered recently by some allocator
changes).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205005 91177308-0d34-0410-b5e6-96231b3b80d8