[CALCITE-3319] AssertionError when reducing decimals

[yanlin-Lynn]

There exist code that throws AssertionError in ReduceDecimalsRule every time it was ran to.

List newOperands = apply(call.getOperands());
if (true) {
// FIXME: Operands are now immutable. Create a new call with
// new operands?
throw new AssertionError();
}

This PR tries to remove this part of code, and add unit test case for the new code path.
Please refer to CALCITE-3319 for full stack trace.

[hsyuan]

+1, but recommend changing the title to AssertionError when reducing decimals so that the title can describe what is the problem this PR is trying to fix.

[yanlin-Lynn]

+1, but recommend changing the title to AssertionError when reducing decimals so that the title can describe what is the problem this PR is trying to fix.

OK. I update the title of this PR and the commit log.

[yanlin-Lynn]

Thanks @hsyuan and @danny0405 !