[pa-combiner] Change worklist iteration to use a more canonical worklist form.

We generally do not use a for loop for worklist iteration since one runs into
weird issues around always needing to recompute the worklist size since it may
change per iteration. In contrast, using the while loop approach that just pops
off the back avoids such implicit weirdness.

Author: gottesmm

lib/SILOptimizer/Utils/PartialApplyCombiner.cpp

@@ -291,11 +291,10 @@ SILInstruction *PartialApplyCombiner::combine() {
   // and look for applies that use it as a callee.
 
   // Worklist of operands.
-  SmallVector<Operand *, 8> uses(pai->getUses());
+  SmallVector<Operand *, 8> worklist(pai->getUses());
 
-  // Uses may grow in this loop.
-  for (size_t useIndex = 0; useIndex < uses.size(); ++useIndex) {
-    auto *use = uses[useIndex];
+  while (!worklist.empty()) {
+    auto *use = worklist.pop_back_val();
     auto *user = use->getUser();
 
     // Recurse through conversions.
@@ -313,7 +312,7 @@ SILInstruction *PartialApplyCombiner::combine() {
       assert(use->get()->getType().castTo<SILFunctionType>() ==
              escapingCalleeTy);
       (void)escapingCalleeTy;
-      llvm::copy(cfi->getUses(), std::back_inserter(uses));
+      llvm::copy(cfi->getUses(), std::back_inserter(worklist));
       continue;
     }
 
@@ -322,7 +321,7 @@ SILInstruction *PartialApplyCombiner::combine() {
       if (mdi->getValue() == use->get() &&
           mdi->getValue()->getType().is<SILFunctionType>() &&
           mdi->getValue()->getType().castTo<SILFunctionType>()->isNoEscape()) {
-        llvm::copy(mdi->getUses(), std::back_inserter(uses));
+        llvm::copy(mdi->getUses(), std::back_inserter(worklist));
       }
       continue;
     }