Remove unnecessary decimal casts for some builtins.

For arithmetic ops, this is an optimization. The Add(decimal,decimal) already handles
the cast as part of the operation.

For binary predicates, the cast is bad and can lead to overflows. The decimal Compare()
function has custom logic to not overflow.

Change-Id: I9f5ad74ea89e9dfa5a3a40c1e07f7e9178bf1d52
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3381
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: jenkins