Use Column.can_cast_safely instead of some ad-hoc dtype functions in .where

[mroeschke]

Description

There were a couple of dedicated functions in python/cudf/cudf/utils/dtypes.py specific to .where that could be subsumed by Column.can_cast_safely.

The minor downside is that we need to cast where's argument to a Column first, but IMO it's probably OK given the deduplication

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[mroeschke]

/merge