GH rust-lang#49: Run GVN again after InstCombine.

Author: eddyb

lib/Transforms/IPO/PassManagerBuilder.cpp

@@ -342,6 +342,8 @@ void PassManagerBuilder::addFunctionSimplificationPasses(
   // Run instcombine after redundancy elimination to exploit opportunities
   // opened up by them.
   addInstructionCombiningPass(MPM);
+  if (OptLevel > 1)
+    MPM.add(createGVNPass(DisableGVNLoadPRE));  // Remove redundancies
   addExtensionsToPM(EP_Peephole, MPM);
   MPM.add(createJumpThreadingPass());         // Thread jumps
   MPM.add(createCorrelatedValuePropagationPass());