This PR fixes the double-free bug mentioned in #911, but is still in draft state, because I suspect it can create memory leaks in certain cases. No review required at the moment.

• The MLIR operation arith.select is frequently created during the canonicalization of scf.if (which, in turn, is created by DaphneDSL if-statements).
• arith.select takes a boolean argument (condition) plus two additional arguments, one of which it selects/returns based on the value of the condition.
• arith.select needs to be taken into account in DAPHNE's object reference counter management, because:
  • We decrease the reference counter of SSA values after their last use as operands (the arith.select could be the last user).
  • We have no clue which of the two last operands of arith.select is returned at run-time.
  • However, the return value is the same runtime object as one of the two latter operands.
  • To ensure the correct reference counter for the result after the arith.select, we need to increase the counter of the result before we decrease the counters of the operands.
• We had already implemented this fix, but applied it only for string operands/results.
• However, we must apply the fix for all operand/result types that have a reference counter at run-time.
• Fixed ManageObjRefPass.
• Added several script-level test cases testing the behavior for matrices, frames, lists, and strings and different settings of the if-then-else in DaphneDSL.