[NFC] Removed unneeded variable.

No need to bind to the StoreBorrowInst to get the source because it's
already bound to the local variable `stored`.  And no need to bind to a
more specific type to find the next instruction.

Author: nate-chandler

lib/SILOptimizer/Transforms/SILMem2Reg.cpp

@@ -407,11 +407,11 @@ beginLexicalLifetimeAfterStore(AllocStackInst *asi, SILInstruction *inst) {
   SILValue stored = inst->getOperand(CopyLikeInstruction::Src);
   SILLocation loc = RegularLocation::getAutoGeneratedLocation(inst->getLoc());
 
-  if (auto *sbi = dyn_cast<StoreBorrowInst>(inst)) {
-    if (isGuaranteedLexicalValue(sbi->getSrc())) {
+  if (isa<StoreBorrowInst>(inst)) {
+    if (isGuaranteedLexicalValue(stored)) {
       return {{stored, SILValue(), SILValue()}, /*isStorageValid*/ true};
     }
-    auto *borrow = SILBuilderWithScope(sbi->getNextInstruction())
+    auto *borrow = SILBuilderWithScope(inst->getNextInstruction())
                        .createBeginBorrow(loc, stored, /*isLexical*/ true);
     return {{stored, borrow, SILValue()}, /*isStorageValid*/ true};
   }
@@ -896,12 +896,12 @@ StackAllocationPromoter::getLiveOutValues(BlockSetVector &phiBlocks,
         LiveValues values = {stored, SILValue(), SILValue()};
         return values;
       }
-      if (auto *sbi = dyn_cast<StoreBorrowInst>(inst)) {
-        if (isGuaranteedLexicalValue(sbi->getSrc())) {
+      if (isa<StoreBorrowInst>(inst)) {
+        if (isGuaranteedLexicalValue(stored)) {
           LiveValues values = {stored, SILValue(), SILValue()};
           return values;
         }
-        auto borrow = cast<BeginBorrowInst>(sbi->getNextInstruction());
+        auto borrow = cast<BeginBorrowInst>(inst->getNextInstruction());
         LiveValues values = {stored, borrow, SILValue()};
         return values;
       }