Subzero: Randomize register assignment.

Randomize the order that registers appear in the free list.  Only
randomize fully "equivalent" registers to ensure no extra spills.

This adds the -randomize-regalloc option.

This is a continuation of https://codereview.chromium.org/456033003/ which Matt owns.

BUG= none
R=jfb@chromium.org

Review URL: https://codereview.chromium.org/807293003

src/IceTargetLoweringX8632.h

diff --git a/src/IceTargetLoweringX8632.h b/src/IceTargetLoweringX8632.h
index 2846ac9..6e2bed9 100644
--- a/src/IceTargetLoweringX8632.h
+++ b/src/IceTargetLoweringX8632.h
@@ -180,6 +180,10 @@
   OperandX8632Mem *getMemoryOperandForStackSlot(Type Ty, Variable *Slot,
                                                 uint32_t Offset = 0);
 
+  void makeRandomRegisterPermutation(
+      llvm::SmallVectorImpl<int32_t> &Permutation,
+      const llvm::SmallBitVector &ExcludeRegisters) const;
+
   // The following are helpers that insert lowered x86 instructions
   // with minimal syntactic overhead, so that the lowering code can
   // look as close to assembly as practical.