Fix regex parsing logic handling of nested quantifiers

[davidwendt]

Description

Fixes the libcudf regex parsing logic when handling nested fixed quantifiers. The logic handles fixed quantifiers by simple repeating the previous instruction. If the previous item is a group (capture or non-capture) that group may also contain an internal fixed quantifier as well.
Found while working on #16730

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[vyasr]

If I understand correctly, all of the in->out substitutions exist because in the nested case out could be filled with additional instructions (lines 746-759), so the sizes now depend on that. Is that right? I think I've grokked this PR but I want to confirm my understanding before I approve.

Out of scope for this PR, but should regcomp.h be renamed to regcomp.hpp? It's not a C header, there are classes in there.

[davidwendt]

That sounds right. Previously it was just copying the input and so did not account for previous expanded (nested) instructions.
I can look into renaming the header. The original implementation came from C source I believe.

[davidwendt]

/merge