Always inline calls with "not obviously legal" pointer args (instead of the opposite).

[eddyb]

• fixes Inliner heuristic for "illegal pointer argument" is incomplete. #1021

Before this PR, the behavior of the inliner wrt pointer types in fn signatures (and pointer args) was to special-case some illegal cases and force inlining. This PR flips some of that on its head, treating everything it's not sure about as illegal, forcing inlining in all the cases not explicitly deemed legal.

Specifically for fixing #1021, the necessary change is obeying the SPIR-V spec about pointer call arguments having to be "memory object declarations" (i.e. an OpVariable, or one of the caller's own OpFunctionParameters, which transitively must be a whole OpVariable etc.).

With this PR, any pointer argument other than an OpVariable (either global or in the caller) or an OpFunctionParameter, will force inlining (even if some of those edge cases may actually be legal, we'll still be correctly conservative).

(The several commits should be reviewed separately, as the first 3 only refactor linker::inline's existing logic)

[ProfLander]

This has been tested with the MRP + (SPIR-T HTML preview) that instigated #1021, and compiles it correctly.

Anecdotally, it's also been tested with the practical case that consumes the cons borrowing trait which highlighted the bug, is functioning as expected there as well 👍