The `unconstrained-less-than` analysis pass does not check if the input is constant

[fegge]

The unconstrained-less-than analysis pass should check if the input evaluates to a constant value before emitting a warning.

In particular, warnings like the following do not make any sense.

warning: Inputs to `LessThan` need to be constrained to ensure that they are non-negative
   ┌─ /.../file.circom:72:28
   │
72 │         lt.in[1] <== 50 + 5;
   │                      ^^^^^^ `50 + 5` needs to be constrained to ensure that it is <= p/2.
   │
   = For more details, see https://github.com/trailofbits/circomspect/blob/main/doc/analysis_passes.md#unconstrained-less-than.