vim-patch:9.2.1096: NFA regexp engine copies submatches slowly without captures (#41904)

Problem:  copy_sub() accounts for a large share of NFA matching time because
          it always calls memmove() to copy submatch positions for every
          state added in addstate(), including the common case of a pattern
          without capturing groups where only the whole match is tracked.
Solution: Copy a single submatch with a direct assignment when only
          subexpression zero is in use, keeping memmove() for patterns with
          more subexpressions (Julien Voisin).

addstate() calls copy_sub() for every state it adds; on a word-boundary
workload copy_sub() and the memmove() it calls together are about 18% of the
NFA matching instructions. A pattern without capturing groups only uses
subexpression zero, so copy_sub() copies a single struct; doing that with a
direct assignment drops the memmove() call from that path.

On a buffer of C code, uncaptured search and substitute patterns (identifiers,
function calls, numbers) get about 6-7%, with no measurable change for patterns
that capture. Not that much, but not that bad for such a small change.

closes: vim/vim#21297

https://github.com/vim/vim/commit/ca57ab28e7f31c90d6af8f134547b455ab88390d

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

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