signatures: generate only one debug breakable line for signatures

[tonycoz]

With the debugger enabled the argcheck op and each argelem op generated to process a function signature would have an associated dbstate op, allowing the debugger to step on that op.

This can be confusing and/or onerous when stepping through code, since for a signature with 7 arguments you'd have 9 steps to get past the prologue of the sub.

Make most of these dbstates into nextstates to make them non-steppable.

Fixes #22602

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