Add bench for the expression chain walk speedup from #6084

Every property fetch / method call resolves its type by walking down to
the chain root to detect a nullsafe operator (NullsafeShortCircuitingHelper),
costing O(N²) walk steps per chain of depth N — with or without an actual
nullsafe operator in the chain. Deep loop-wrapped plain chains make that
walk dominate: 3.71s -> 3.14s wall (-15%), -18% user CPU from the
recursion-to-loop rewrite. The real-world counterpart is Symfony
TreeBuilder fluent chains (300+ calls in one statement) in Sylius bundle
Configuration classes, which dropped up to 23% per file.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016szvNF5RXhACdfMQNc6DVL

Author: ondrejmirtes