Extend TargetPassConfig to allow running only a subset of the normal passes.

This is still a work in progress but I believe it is currently good enough
to fix PR13122 "Need unit test driver for codegen IR passes".  For example,
you can run llc with -stop-after=loop-reduce to have it dump out the IR after
running LSR.  Serializing machine-level IR is not yet supported but we have
some patches in progress for that.

The plan is to serialize the IR to a YAML file, containing separate sections
for the LLVM IR, machine-level IR, and whatever other info is needed.  Chad
suggested that we stash the stop-after pass in the YAML file and use that
instead of the start-after option to figure out where to restart the
compilation.  I think that's a great idea, but since it's not implemented yet
I put the -start-after option into this patch for testing purposes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@159570 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/NVPTX/NVPTXTargetMachine.h

@@ -48,12 +48,6 @@ class NVPTXTargetMachine : public LLVMTargetMachine {
   //                            bool DisableVerify, MCContext *&OutCtx);
 
 public:
-  //virtual bool addPassesToEmitFile(PassManagerBase &PM,
-  //                                 formatted_raw_ostream &Out,
-  //                                 CodeGenFileType FileType,
-  //                                 CodeGenOpt::Level OptLevel,
-  //                                 bool DisableVerify = true) ;
-
   NVPTXTargetMachine(const Target &T, StringRef TT, StringRef CPU,
                      StringRef FS, const TargetOptions &Options,
                      Reloc::Model RM, CodeModel::Model CM,