[TVM][ARITH] integer set analysis does not handle Select

[derisavi]

When faced with Select operator, IntSetEvaluator will do the default thing which is to return IntSet::everything() which is too conservative.
The right thing to do is to return the union of IntSets of true and false expressions.

[tqchen]

This sounds like something we can directly send a patch to fix, we can also consolidate it to #2588

I have already done so for some of the analysis in #2668

[derisavi]

I already have a fix (5 lines of code) for this small bug. I was going to send a PR but I'm open to suggestion on what do with it (or even not to do anything with it and wait for a PR for #2588)