[PATCH] Add a lock() function in PassRegistry to speed up multi-thread synchronization.

[Mehdi AMINI]

================
Comment at: lib/IR/PassRegistry.cpp:52
@@ +51,3 @@
+  if (needMutex)
+    Lock.unlock_shared();
+  return PI;
----------------
It is unfortunate to lose RAII, is there a good reason why SmartScopedReader cannot take an extra parameter to be constructed without locking? (like std::unique_lock())

http://reviews.llvm.org/D7787

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