[InstCombine] add helper for "is desirable int type"; NFC

This splits out the logic from shouldChangeType() that
currently allows 8/16/32-bit transforms even if those
types are not listed as legal in the data layout.

This could be useful as a predicate for vector
insert/extract transforms.

Note that this leaves the subsequent checks in
shouldChangeType() unchanged. We may want to merge
the checks for i1 and/or "ToLegal" into "isDesirable",
but that may alter existing transforms.