Treat mid-block labels the same as terminators when building the

MachineInstr scheduling DAG, meaning they implicitly depend on all
preceding defs. This fixes Benchmarks/Shootout-C++/except and
Regression/C++/EH/simple_rethrow in
  -relocation-model=pic -disable-post-RA-scheduler=false
mode.

llvm-svn: 59747