AMDGPU: Fix iterator error when lowering SI_END_CF

If the instruction is the last in the block, there is no next
instruction but the iteration still needs to look at the new block.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369203 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed