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

[muellerj2]

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.

[muellerj2]

I guess nodes of type _N_endif count as well, but I suspect that one is even more intricate to remove than the other three no-ops.