[PATCH] [SCEV] Extract and strengthen isHighCostExpansion.

[Sanjoy Das]

> It's kind of nice to move code in a separate change from updating it, so
> that it's easier to see what actually changed.

Done.  The NFCI part is in D8995.

> We don't usually repeat the function name in the doc comment for new
> code.

I went with it since that is the convention in the rest of the file.
If it is okay with you, I'd prefer fixing this by submitting this
change as is and fixing the whole ScalarEvolutionExpander.h file at
once in an NFC commit.  Also, I could not easily find this in
CodingStandards.html -- should this clause be added there?

-- Sanjoy