Randomize instruction scheduling to increase security against ROP attacks (clang)

[yln]

Summary

Modifies the selection DAG scheduler to randomize instruction selection to introduce diversity with the goal of increasing security against most return-oriented programming attacks.

Command line options:

-sched-randomize // Enable the schedule randomizer.
-sched-randomize-percentage=X // X% of instructions are scheduled randomly (default: 50%, requires -sched-randomize).

This is the clang part of the patch.
LLVM part: D3394

[jfb]

Moving this forward, this LGTM assuming the other changes make it in too.

lib/Driver/Tools.cpp

 
   if (Args.hasArg(options::OPT_fdiversify)) {
     CmdArgs.push_back("-nop-insertion");
+
+    CmdArgs.push_back("-backend-option");
+    CmdArgs.push_back("-sched-randomize");
   }
 
   if (Arg *A = Args.getLastArg(options::OPT_mrestrict_it,