[llvm][mustache] Avoid redundant saves in accessor splitting

The splitMustacheString function was saving StringRefs that
were already backed by an arena-allocated string. This was
unnecessary work. This change removes the redundant
Ctx.Saver.save() call.

This optimization provides a small but measurable performance
improvement on top of the single-pass tokenizer, most notably
reducing branch misses.

  Metric         | Baseline | Optimized | Change
  -------------- | -------- | --------- | -------
  Time (ms)      | 35.77    | 35.57     | -0.56%
  Cycles         | 35.16M   | 34.91M    | -0.71%
  Instructions   | 85.77M   | 85.54M    | -0.27%
  Branch Misses  | 113.9K   | 111.9K    | -1.76%
  Cache Misses   | 237.7K   | 242.1K    | +1.85%

Author: ilovepi

llvm/lib/Support/Mustache.cpp

@@ -51,7 +51,7 @@ static Accessor splitMustacheString(StringRef Str, MustacheContext &Ctx) {
       std::tie(Part, Str) = Str.split('.');
       // Each part of the accessor needs to be saved to the arena
       // to ensure it has a stable address.
-      Tokens.push_back(Ctx.Saver.save(Part.trim()));
+      Tokens.push_back(Part.trim());
     }
   }
   // Now, allocate memory for the array of StringRefs in the arena.