[FileCheck] Avoid repeated hash lookups (NFC) (#131553)

Author: kazutakahirata

llvm/lib/FileCheck/FileCheck.cpp

@@ -1010,8 +1010,10 @@ bool Pattern::parsePattern(StringRef PatternStr, StringRef Prefix,
         // Handle substitution of string variables that were defined earlier on
         // the same line by emitting a backreference. Expressions do not
         // support substituting a numeric variable defined on the same line.
-        if (!IsNumBlock && VariableDefs.find(SubstStr) != VariableDefs.end()) {
-          unsigned CaptureParenGroup = VariableDefs[SubstStr];
+        decltype(VariableDefs)::iterator It;
+        if (!IsNumBlock &&
+            (It = VariableDefs.find(SubstStr)) != VariableDefs.end()) {
+          unsigned CaptureParenGroup = It->second;
           if (CaptureParenGroup < 1 || CaptureParenGroup > 9) {
             SM.PrintMessage(SMLoc::getFromPointer(SubstStr.data()),
                             SourceMgr::DK_Error,