[NewGVN] Add command-line option to control the generation of phi-of-ops (disable by default)

[mcrosier]

Summary

Useful for testing and debugging. Disabled by default, per Danny's request.

[• dberlin]

I think where i come down on this is: disabled by default , and modify the existing test arguments to enable it, so that you don't have to xfail or modify a ton of tests :)
(then when it's fixed we can just remove the test arguments again).
But i'm also not opposed to doing the test updating.

[mcrosier]

-Disable this feature by default.
-Update test RUN commands due to change.

[mcrosier]

r310594

lib/Transforms/Scalar/NewGVN.cpp

 DEBUG_COUNTER(PHIOfOpsCounter, "newgvn-phi",
               "Controls which instructions we create phi of ops for")
 // Currently store defining access refinement is too slow due to basicaa being
 // egregiously slow.  This flag lets us keep it working while we work on this
 // issue.
 static cl::opt<bool> EnableStoreRefinement("enable-store-refinement",
                                            cl::init(false), cl::Hidden);
 
+/// Currently, the generation "phi of ops" can result in correctness issues.
+/// This flag lets us keep it working while we work on these issues.
+static cl::opt<bool> EnablePhiOfOps("enable-phi-of-ops", cl::init(true),
+                                    cl::Hidden);
+
 //===----------------------------------------------------------------------===//
 //                                GVN Pass
 //===----------------------------------------------------------------------===//
 
 // Anchor methods.
 namespace llvm {
 namespace GVNExpression {
 Expression::~Expression() = default;
void NewGVN::addPhiOfOps(PHINode *Op, BasicBlock *BB,
   InstrDFS[Op] = InstrToDFSNum(ExistingValue);
   AllTempInstructions.insert(Op);
   PHIOfOpsPHIs[BB].insert(Op);
   TempToBlock[Op] = BB;
   RealToTemp[ExistingValue] = Op;
 }
 
 static bool okayForPHIOfOps(const Instruction *I) {
+  if (!EnablePhiOfOps)
+    return false;
   return isa<BinaryOperator>(I) || isa<SelectInst>(I) || isa<CmpInst>(I) ||
          isa<LoadInst>(I);
 }
 
 // When we see an instruction that is an op of phis, generate the equivalent phi
 // of ops form.
 const Expression *
 NewGVN::makePossiblePhiOfOps(Instruction *I,