DRY out data suffix handling

[Rich-Harris]

A thought I had while reviewing #7416 — our data suffix handling is a bit raggedy, and would maybe be nicer if we encapsulated the repetitious logic into helper functions.

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[dummdidumm]

One suggestion, but I'm fine with it either way

[dummdidumm]

should we use has_data_suffix here and make this a noop in case it's not the suffix? Would reduce code at one call site.

[Rich-Harris]

I'm not a huge fan of functions that are 'forgiving' about their inputs — it adds modest convenience (albeit at a single call site in our case, IOW it's just moving the logic around rather than reducing repetition) at the cost of greater uncertainty and wasted computation (however negligible)