Fix a nasty bug in exprIsExpandable
This bug has been lurking for ages: Trac #13155 The important semantic change is to ensure that exprIsExpandable returns False for primop calls. Previously exprIsExpandable used exprIsCheap' which always used primOpIsCheap. I took the opportunity to combine the code for exprIsCheap' (two variants: exprIsCheap and exprIsExpandable) with that for exprIsWorkFree. Result is simpler, tighter, easier to understand. And correct (at least wrt this bug)!
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