[Scalar] Use std::optional in InductiveRangeCheckElimination.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

Author: kazutakahirata

llvm/lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp

@@ -93,6 +93,7 @@
 #include <cassert>
 #include <iterator>
 #include <limits>
+#include <optional>
 #include <utility>
 #include <vector>
 
@@ -547,8 +548,8 @@ class LoopConstrainer {
   // space of the main loop.
 
   struct SubRanges {
-    Optional<const SCEV *> LowLimit;
-    Optional<const SCEV *> HighLimit;
+    std::optional<const SCEV *> LowLimit;
+    std::optional<const SCEV *> HighLimit;
   };
 
   // Compute a safe set of limits for the main loop to run in -- effectively the