C#: Prevent bad magic in getAnAccess

[Mathias Vorreiter Pedersen (MathiasVP)]

We were seeing bad magic like this on dotnet/roslyn:

[2026-09-17 18:47:43] Evaluated non-recursive predicate m#Variable::Variable.getAnAccess/0#340bbb9f#fb@8b5f56i8 in 441814ms (evaluation was halted due to CancellationException).
Evaluated relational algebra for predicate m#Variable::Variable.getAnAccess/0#340bbb9f#fb@8b5f56i8 with tuple counts:
             7149985     ~3%    {1} r1 = SCAN `Expr::Expr.getEnclosingCallable/0#dispred#f6dee370` OUTPUT In.0
             7149985     ~3%    {1}    | STREAM DEDUP
                            
        269552176500  ~1576%    {2} r2 = JOIN `DataFlowPublic::ExprNode.getExpr/0#dispred#ad3278f4_1#join_rhs` WITH @field CARTESIAN PRODUCT OUTPUT Rhs.0, Lhs.0
                   0     ~0%    {1}    | JOIN WITH Remote::AspNetQueryStringMember#dee4d488 ON FIRST 1 OUTPUT Lhs.1
                            
             7149985     ~3%    {1} r3 = r1 UNION r2
                                return r3