The `signal-assignment` analysis pass should not emit warnings for constrained signals

[fegge]

If the assigned signal is constrained elsewhere in the circuit, it is enough to emit an informational finding.

In particular, the following should not raise a warning.

warning: Using the signal assignment operator `<--` does not constrain the assigned signal.
   ┌─ /.../file.circom:72:9
   │
72 │         m[i] <-- out[0][i];
   │         ^^^^^^^^^^^^^^^^^^^^^^^^^ The assigned signal `m[i]` is not constrained here.
73 │         mRangeCheck[i] = Num2Bits(n);
74 │         mRangeCheck[i].in <== m[i];
   │         -------------------------- The signal `q[i]` is constrained here.
   │
   = For more details, see https://github.com/trailofbits/circomspect/blob/main/doc/analysis_passes.md#signal-assignment.