Subzero: Allow delaying Phi lowering until after register allocation.

src/IceRegAlloc.cpp

Index: src/IceRegAlloc.cpp
diff --git a/src/IceRegAlloc.cpp b/src/IceRegAlloc.cpp
index 1b4d0c249d0d872f2aecb705081dce83e25a04c8..d3fae45e909c6b2544bd56dc2642b055d633be32 100644
--- a/src/IceRegAlloc.cpp
+++ b/src/IceRegAlloc.cpp
@@ -105,12 +105,15 @@ void LinearScan::scan(const llvm::SmallBitVector &RegMaskFull) {
     for (Variable *Var : Vars) {
       // Explicitly don't consider zero-weight variables, which are
       // meant to be spill slots.
-      if (Var->getWeight() == RegWeight::Zero)
+      if (Var->getWeight() == RegWeight::Zero) {
+        Var->setNeedsStackSlot();
         continue;
+      }
       // Don't bother if the variable has a null live range, which means
       // it was never referenced.
       if (Var->getLiveRange().isEmpty())
         continue;
+      Var->setNeedsStackSlot();
       Var->untrimLiveRange();
       Unhandled.push_back(Var);
       if (Var->hasReg()) {