[llvm][mustache] Avoid redundant saves in accessor splitting (#159197)

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%

0 files changed, 0 insertions, 0 deletions