mirpasses: split assignments with MIR pass

[zerbina]

Summary

• use MIR pass to split assignments involving potentially raising calls
• for the C backend, the compiler employing RVO no longer affects
  observable behaviour

Details

• add the splitAssignments MIR pass and enable it for the C backend
• the condition of when to split assignments is the same as
  ccgcalls.isHarmlessStore, with the difference that
  splitAssignments also considers projections of locals when
  optimizing away a split (not only unprojected locals)
• ccgcalls can now assume that all assignments are safe; everything
  related to assignment splitting is removed

Observable stores

When the RHS of an assignment was an RVO-using call, the assignment was
not split, resulting in an "observable store" if the call raised an
exception after having modified its result variable.

The splitAssignments call splits all assignments, incurring an
additional memory copy for RVO-using calls but also making the
behaviour "correct". reportObservableStore is obsolete and thus
removed.

[saem]

/merge

[github-actions]

Merge requested by: @saem

Contents after the first section break of the PR description has been removed and preserved below:

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Notes for Reviewers

• the return-value optimization not affecting observable behaviour is preferable over less extra memory copies, in my opinion
• this is a preparation for WIP: rewrite the C code generator #1333