`<regex>`: Avoid generating no-op NFA nodes

The NFA nodes of type _N_nop, _N_group and _N_end_group do not result in any changes to the match state in the matcher. However, processing them still requires some time. Going by the benchmarks in #5939, it seems about 2.5 ns are spent on my machine whenever when such a node is encountered by the matcher; see, e.g., the 37 % (relative) / 470 ns (absolute) increase in running time when matching the pattern (?:a)* to 100 a's compared to a*.

Note that these NFA nodes do usually serve some purpose during regex parsing, so resolving this issue requires some careful changes to the parser logic.

Start with the regex parser and matcher logic described in the issue, then review the benchmark referenced in #5939, especially the (?:a)* versus a* case. Trace how _N_nop, _N_group, and _N_end_group nodes are produced and processed. Done means preserving regex behavior while avoiding unnecessary nodes and demonstrating reduced matching overhead.