[BOLT][NFC] Fix for a dangling reference UB (llvm#163344)

Fix UB caused by accessing the top element of the stack via a dangling
reference after a call to .pop() This is tripping static analysis.

No functional changes. Performance impact is negligible, but alt.
implementation of the fix is possible if needed.

Testing: Both functional and unit tests are passing.

Author: noclowns

bolt/lib/Passes/FrameAnalysis.cpp

@@ -198,7 +198,7 @@ class FrameAccessAnalysis {
         if (CFIStack.empty())
           dbgs() << "Assertion is about to fail: " << BF.getPrintName() << "\n";
         assert(!CFIStack.empty() && "Corrupt CFI stack");
-        std::pair<int64_t, uint16_t> &Elem = CFIStack.top();
+        std::pair<int64_t, uint16_t> Elem = CFIStack.top();
         CFIStack.pop();
         CfaOffset = Elem.first;
         CfaReg = Elem.second;

bolt/lib/Passes/ShrinkWrapping.cpp

@@ -402,7 +402,7 @@ void StackLayoutModifier::classifyCFIs() {
         break;
       case MCCFIInstruction::OpRestoreState: {
         assert(!CFIStack.empty() && "Corrupt CFI stack");
-        std::pair<int64_t, uint16_t> &Elem = CFIStack.top();
+        std::pair<int64_t, uint16_t> Elem = CFIStack.top();
         CFIStack.pop();
         CfaOffset = Elem.first;
         CfaReg = Elem.second;