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/CodeGen/Passes.cpp

@@ -213,7 +213,8 @@ TargetPassConfig::~TargetPassConfig() {
 // Out of line constructor provides default values for pass options and
 // registers all common codegen passes.
 TargetPassConfig::TargetPassConfig(TargetMachine *tm, PassManagerBase &pm)
-  : ImmutablePass(ID), PM(&pm), TM(tm), Impl(0), Initialized(false),
+  : ImmutablePass(ID), PM(&pm), StartAfter(0), StopAfter(0),
+    Started(true), Stopped(false), TM(tm), Impl(0), Initialized(false),
     DisableVerify(false),
     EnableTailMerge(true) {
 
@@ -271,11 +272,22 @@ AnalysisID TargetPassConfig::getPassSubstitution(AnalysisID ID) const {
   return I->second;
 }
 
-/// Add a pass to the PassManager.
+/// Add a pass to the PassManager if that pass is supposed to be run.  If the
+/// Started/Stopped flags indicate either that the compilation should start at
+/// a later pass or that it should stop after an earlier pass, then do not add
+/// the pass.  Finally, compare the current pass against the StartAfter
+/// and StopAfter options and change the Started/Stopped flags accordingly.
 void TargetPassConfig::addPass(Pass *P) {
   assert(!Initialized && "PassConfig is immutable");
 
-  PM->add(P);
+  if (Started && !Stopped)
+    PM->add(P);
+  if (StopAfter == P->getPassID())
+    Stopped = true;
+  if (StartAfter == P->getPassID())
+    Started = true;
+  if (Stopped && !Started)
+    report_fatal_error("Cannot stop compilation after pass that is not run");
 }
 
 /// Add a CodeGen pass at this point in the pipeline after checking for target