[StackNesting] NFC: Added helper.

Just pull the notion that the alloc corresponds to operand 0 into a
helper method.

Author: nate-chandler

include/swift/SILOptimizer/Utils/StackNesting.h

@@ -153,10 +153,17 @@ class StackNesting {
   /// Returns the location bit number for a stack deallocation instruction.
   int bitNumberForDealloc(SILInstruction *DeallocInst) {
     assert(DeallocInst->isDeallocatingStack());
-    auto *AllocInst = cast<SingleValueInstruction>(DeallocInst->getOperand(0));
+    auto *AllocInst = getAllocForDealloc(DeallocInst);
     return bitNumberForAlloc(AllocInst);
   }
 
+  /// Returns the stack allocation instruction for a stack deallocation
+  /// instruction.
+  SingleValueInstruction *getAllocForDealloc(SILInstruction *Dealloc) const {
+    return cast<SingleValueInstruction>(
+        Dealloc->getOperand(0)->getDefiningInstruction());
+  }
+
   /// Insert deallocations at block boundaries.
   Changes insertDeallocsAtBlockBoundaries();
 

lib/SILOptimizer/Utils/StackNesting.cpp

@@ -44,7 +44,7 @@ void StackNesting::setup() {
 
         BI.StackInsts.push_back(Alloc);
       } else if (I.isDeallocatingStack()) {
-        auto *AllocInst = cast<SingleValueInstruction>(I.getOperand(0));
+        auto *AllocInst = getAllocForDealloc(&I);
         if (!BI.StackInsts.empty() && BI.StackInsts.back() == AllocInst) {
           // As an optimization, we ignore perfectly nested alloc-dealloc pairs
           // inside a basic block.
@@ -374,7 +374,7 @@ void StackNesting::dump() const {
         dumpBits(StackLocs[BitNr].AliveLocs);
         llvm::dbgs() << ",     " << *StackInst;
       } else if (StackInst->isDeallocatingStack()) {
-        auto *AllocInst = cast<SingleValueInstruction>(StackInst->getOperand(0));
+        auto *AllocInst = getAllocForDealloc(StackInst);
         int BitNr = StackLoc2BitNumbers.lookup(AllocInst);
         llvm::dbgs() << "  dealloc for #" << BitNr << "\n"
                         "    " << *StackInst;