C++: Fix more asExpr duplication

[MathiasVP]

When we check for whether an instruction represents the "fully converted expression" we ask for the expression associated with the instruction, and require that no "later" instruction associated with the same expression exists. This last part is done via the integer n in the charpred that looks like:

exists(Expr e, int n |
  exprNodeShouldBeInstruction(this, e, n) and
  not exprNodeShouldBe(e, n + 1)
)

However, it may actually be the case that one instruction is associated with one conversion of an instruction, but a later instruction is associated with another conversion of the same expression. To account for this, instead of requiring that there doesn't exist another instruction with a larger n and the same instruction, we ask whether there exists an instruction with a larger n and a conversion of the same unconverted instruction. That is, we now do:

exists(Expr e, int n |
  exprNodeShouldBeInstruction(this, e, n) and
  not exists(Expr conv |
    exprNodeShouldBe(conv, n + 1) and
    conv.getUnconverted() = e.getUnconverted()
  )
)

and this fixes the remaining dataflow node duplication in our asExpr tests 🎉