Fix invalid deterministic fform evaluation

[bvdmitri]

This PR fixes cases where deterministic evaluation was invalid. E.g. in cases like

@model function broken_beta_bernoulli(y)
        θ ~ Matrix([1.0 0.0; 0.0 1.0])
        for i in eachindex(y)
            y[i] ~ Bernoulli(θ)
        end
    end

The Matrix([1.0 0.0; 0.0 1.0]) was evaluated and discarded. This PR fixes that, by allowing the evaluation of rhs only if the lhs is an anonymous variable. Such that this is still possible

θ ~ MvNormal(mean = zeros(2), covariance = Matrix([1.0 0.0; 0.0 1.0]))

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.90%. Comparing base (54eb171) to head (21c7478).
Report is 13 commits behind head on dev-4.0.0.

Additional details and impacted files

@@              Coverage Diff              @@
##           dev-4.0.0     #185      +/-   ##
=============================================
+ Coverage      91.85%   91.90%   +0.04%     
=============================================
  Files             15       15              
  Lines           1842     1841       -1     
=============================================
  Hits            1692     1692              
+ Misses           150      149       -1     

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