Remove the nasty LABEL hack with a much less evil one. Now llvm.dbg.func.start...

Remove the nasty LABEL hack with a much less evil one. Now llvm.dbg.func.start implies a stoppoint is set. SelectionDAGISel records a new source line but does not create a ISD::LABEL node for this special stoppoint. Asm printer will magically print this label. This ensures nothing is emitted before.

llvm-svn: 46635