Allow CodeGen (llc) command line options to work as expected.

The llc command line options for enabling/disabling passes are local to CodeGen/Passes.cpp. This patch associates those options with standard pass IDs so they work regardless of how the target configures the passes.

A target has two ways of overriding standard passes:
1) Redefine the pass pipeline (override TargetPassConfig::add%Stage)
2) Replace or suppress individiual passes with TargetPassConfig::substitutePass.

In both cases, the command line options associated with the pass override the target default.

For example, say a target wants to disable machine instruction scheduling by default:

- The target calls disablePass(MachineSchedulerID) but otherwise does not override any TargetPassConfig methods.

- Without any llc options, no scheduler is run.

- With -enable-misched, the standard machine scheduler is run and honors the -misched=... flag to select the scheduler variant, which may be used for performance evaluation or testing.

Sorry overridePass is ugly. I haven't thought of a better way without replacing the cl::opt framework. I hope to do that one day...

I haven't figured out why CodeGen uses char& for pass IDs. AnalysisID is much easier to use and less bug prone. I'm using it wherever I can for internal implementation. Maybe later we can change the global pass ID definitions as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@150563 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/Passes.h

@@ -40,6 +40,20 @@ class PassConfigImpl;
 /// This is an ImmutablePass solely for the purpose of exposing CodeGen options
 /// to the internals of other CodeGen passes.
 class TargetPassConfig : public ImmutablePass {
+public:
+  /// Pseudo Pass IDs. These are defined within TargetPassConfig because they
+  /// are unregistered pass IDs. They are only useful for use with
+  /// TargetPassConfig APIs to identify multiple occurrences of the same pass.
+  ///
+
+  /// EarlyTailDuplicate - A clone of the TailDuplicate pass that runs early
+  /// during codegen, on SSA form.
+  static char EarlyTailDuplicateID;
+
+  /// PostRAMachineLICM - A clone of the LICM pass that runs during late machine
+  /// optimization after regalloc.
+  static char PostRAMachineLICMID;
+
 protected:
   TargetMachine *TM;
   PassManagerBase ±