Treat the label for the first @llvm.dbg.stoppoint the same way as the dbg_func_start...

Treat the label for the first @llvm.dbg.stoppoint the same way as the dbg_func_start label. Make sure nothing else is inserted before them.
Note this solution might be somewhat fragile since ISD::LABEL may be used for other
purposes. If that ends up to be an issue, we may need to introduce a different node
for debug labels.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@46571 91177308-0d34-0410-b5e6-96231b3b80d8