C++: Fix forex recursion in sign analysis

[MathiasVP]

It's well-known that recursion through forall/forex may perform poorly. Apparently we do recursion through forex in the sign analysis library (which is part of the new range analysis library for C++). This results in some unfortunate tuple counts because the "range" part of the forall/forex gets joined with itself:

Tuple counts for _RangeAnalysisImpl::SignAnalysis::zeroBound/3#c119dcdb_RangeAnalysisImpl::SignAnalysis::zeroBound/3#__#antijoin_rhs/3@i4#L3#76559w2g after 1m28s:
  256000    ~900%         {4} r1 = JOIN `RangeAnalysisImpl::SignAnalysis::zeroBoundOk/3#a6688e7a#prev_delta` WITH num#Sign::TZero#65a6fcab CARTESIAN PRODUCT OUTPUT Lhs.0, Lhs.1 'arg1', Lhs.2 'arg2', Rhs.0
  251000    ~1835%        {3}    | JOIN WITH `RangeAnalysisImpl::SignAnalysis::zeroBound/3#c119dcdb` ON FIRST 3 OUTPUT Lhs.1 'arg1', Lhs.2 'arg2', Lhs.3 'arg0'
  692772000 ~2073%        {4}    | JOIN WITH `RangeAnalysisImpl::SignAnalysis::zeroBound/3#c119dcdb_120#join_rhs` ON FIRST 2 OUTPUT Rhs.2, Lhs.0 'arg1', Lhs.1 'arg2', Lhs.2 'arg0'
                          {4}    | AND NOT `RangeAnalysisImpl::SignAnalysis::zeroBoundOk/3#a6688e7a#prev`(FIRST 3)
  64314500  ~1238106%     {3}    | SCAN OUTPUT In.3 'arg0', In.1 'arg1', In.2 'arg2'

Luckily, there's a standard way to work around this problem by rewriting the forall/forex to recursion over integers. This PR does just that. This results in much better RA.

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