[mlir] Use llvm::SmallVector::pop_back_val() (NFC) (#136452)

Author: kazutakahirata

mlir/lib/Analysis/Liveness.cpp

@@ -198,8 +198,7 @@ Liveness::OperationListT Liveness::resolveLiveness(Value value) const {
 
   while (!toProcess.empty()) {
     // Get block and block liveness information.
-    Block *block = toProcess.back();
-    toProcess.pop_back();
+    Block *block = toProcess.pop_back_val();
     const LivenessBlockInfo *blockInfo = getLiveness(block);
 
     // Note that start and end will be in the same block.

mlir/lib/Rewrite/ByteCode.cpp

@@ -1198,8 +1198,7 @@ class ByteCodeExecutor {
   /// Pops a code iterator from the stack, returning true on success.
   void popCodeIt() {
     assert(!resumeCodeIt.empty() && "attempt to pop code off empty stack");
-    curCodeIt = resumeCodeIt.back();
-    resumeCodeIt.pop_back();
+    curCodeIt = resumeCodeIt.pop_back_val();
   }
 
   /// Return the bytecode iterator at the start of the current op code.