Simplify "1 = 1" to true, "1 > 2" to false

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.

https://github.com/apache/calcite/pull/376