[PATCH] compile time improvement for MachineSink

[Daniel Berlin]

REPOSITORY
  rL LLVM

================
Comment at: lib/CodeGen/MachineSink.cpp:300
@@ +299,3 @@
+  }
+  RegsToClearKillFlags.clear();
+
----------------
```
for (auto Reg : RegsToClearKillFlags)
  MRI->clearKillFlags(Reg);

RegsToClearKillFlags.clear()
```

http://reviews.llvm.org/D9719

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