SSA: Tolerate multiple variables being read at the same CFG node

[asgerf]

Fixes an issue with the SSA data-flow integration library that occurs when multiple variables are read at the same CFG node.

This situation happens when it is instantiated from the VariableCapture library. It also happens in unified due to how we use synthetic reads to mark post-update positions.

The assumption has a subtle presence in this bit of code:

    private class ReadNodeImpl extends ExprNodeImpl {
      private BasicBlock bb_;
      private int i_;

      ReadNodeImpl() {
        variableRead(bb_, i_, v_, true) and
        this.getExpr().hasCfgNode(bb_, i_)
      }

Since this is only bound to the expression, the variableRead join can fan out to multiple variables when not uniquely determined by the CFG node. Even if Expr is constructed to be unique to the variable, the correlation is forgotten in this join and they get mixed up anyway. The fix was to make TExprNode unique to a specific variable.

The real culprit is the Expr class from the data-flow integration input. I propose we remove Expr entirely and replace ExprNode with ReadNode: a canonical representative for a (bb,i,v) triple from the variableRead input, and likewise for PostUpdateNode. But such a change is too large for this PR as it requires language-specific refactorings. But the fix is essentially a precursor to this solution: when Expr is a CFG node, ReadNode is effectively a (bb,i,v) triple as it rightfully should be.