The RexUtil.simplify method should handle literal comparison simplification. From an email on the calcite-dev mailing list:
Calcite doesn't seem to optimize away the literal comparison literal case
with RexUtil.simplify. In my understanding any literal comparison literal
results in a simple TRUE/FALSE result.
I'm not sure this is valid in the general case, but I put together a simple
example of doing this on the RexUtil simplifyCall.