vim-patch:9.2.1029: NFA regexp matching is slower than necessary (#41611)

Problem:  NFA regexp matching is slower than necessary because addstate()
          tracks the recursion depth in a static variable.
Solution: Pass the recursion depth as a function parameter so it stays in a
          register instead of being spilled to memory around every recursive
          call (Julien Voisin).

addstate() is called very frequently and recursively. Storing the depth
counter in a static variable forces the compiler to reload and store it in
memory around each recursive call, since it cannot prove the recursion does
not modify it. Passing it as a parameter keeps it in a register and drops
the increment/decrement bookkeeping on every return path. The recursion
limit behavior is unchanged.

This reduces the instruction count of NFA matching by roughly 4% on
state-heavy patterns. Benchmarking was done with something like:

```
set re=0
let s:word = repeat('abc123 def456 ghijk_lmnop QRSTUV wxyz0 ', 4000)
for i in range(120)
  let s = s:word | let s = substitute(s, '\%#=2\(\a\+\)\(\d\+\)', '\2\1', 'g')
  let s = s:word | let s = substitute(s, '\%#=2\a\+\d\+', 'X', 'g')
  call matchstr(s:word, '\%#=2\(\a\|\d\|_\)\{3,}')
endfor
qa!
```

and `taskset -c 7 perf stat -e instructions ./vim -u NONE -N -X -es -S /tmp/nfabench/count.vim`

closes: vim/vim#21202

https://github.com/vim/vim/commit/0e47c9612edfbcdd299dfacc78521d3ead88baa7

Co-authored-by: Julien Voisin <julien.voisin@dustri.org>

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