[GlobalISel] Use std::optional in LegalizerHelper.cpp (NFC)

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716
diff --git a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
index 6ea4f6c..b0bba68 100644
--- a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -34,6 +34,7 @@
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Target/TargetMachine.h"
 #include <numeric>
+#include <optional>
 
 #define DEBUG_TYPE "legalizer"
 
@@ -1842,7 +1843,7 @@
                                            LLT WideTy) {
   unsigned Opcode;
   unsigned ExtOpcode;
-  Optional<Register> CarryIn;
+  std::optional<Register> CarryIn;
   switch (MI.getOpcode()) {
   default:
     llvm_unreachable("Unexpected opcode!");