[CALCITE-6453] Simplify casts which are result of constant reduction

[kasakrisz]

In RexSimplify.simplifyCast after constant reduction return the reduced expressions operand as result if the reduced expression and it's operand type is equal but different in nullability.

[sonarqubecloud[bot]]

Quality Gate passed

Issues
5 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[asolimando]

I see several approvals (including committers) and there seems to be no pending issues, can this be merged?

[mihaibudiu]

I have no other comments.

[kasakrisz]

Thanks you guys for the review and merging this patch.