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.

llvm-svn: 150563