Move the Target way of overriding DAG Scheduler to a target hook

Summary:
The previous way of overriding it was relying on calling "setDefault"
on the global registry, which implies global mutable state.

Reviewers: echristo, atrick

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11538

From: Mehdi Amini <mehdi.amini@apple.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243388 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/SchedulerRegistry.h

@@ -52,12 +52,6 @@ public:
   static RegisterScheduler *getList() {
     return (RegisterScheduler *)Registry.getList();
   }
-  static FunctionPassCtor getDefault() {
-    return (FunctionPassCtor)Registry.getDefault();
-  }
-  static void setDefault(FunctionPassCtor C) {
-    Registry.setDefault((MachinePassCtor)C);
-  }
   static void setListener(MachinePassRegistryListener *L) {
     Registry.setListener(L);
   }