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/X86.h
--- lib/Target/X86/X86.h.orig
+++ lib/Target/X86/X86.h
@@ -122,6 +122,10 @@ FunctionPass *createX86FixupBWInsts();
 /// to another, when profitable.
 FunctionPass *createX86DomainReassignmentPass();
 
+/// Return a Machine Function pass that attempts to replace
+/// ROP friendly instructions with alternatives.
+FunctionPass *createX86FixupGadgetsPass();
+
 /// This pass replaces EVEX encoded of AVX-512 instructiosn by VEX
 /// encoding when possible in order to reduce code size.
 FunctionPass *createX86EvexToVexInsts();