Add a clang pass that identifies potential ROP gadgets and replaces ROP
friendly instructions with safe alternatives. This initial commit fixes
3 instruction forms that will lower to include a c3 (return) byte.
Additional problematic instructions can be fixed incrementally using
this framework.

Index: lib/Target/X86/X86TargetMachine.cpp
--- lib/Target/X86/X86TargetMachine.cpp.orig
+++ lib/Target/X86/X86TargetMachine.cpp
@@ -546,6 +546,7 @@ void X86PassConfig::addPreEmitPass() {
   addPass(createX86EvexToVexInsts());
   addPass(createX86DiscriminateMemOpsPass());
   addPass(createX86InsertPrefetchPass());
+  addPass(createX86FixupGadgetsPass());
   addPass(createX86InsertX87waitPass());
 }