PATCH: Fix sign-extension of bignums

[Mark Mitchell]

Mark Mitchell wrote:

> Why can't we just unconditionally sign-extend the value in emit_expr, 
> if X_unsigned is false?

With Alan's explanation, I know see that's exactly what we were trying 
to do, and I now understand exactly what you're saying.

Could we generate a bignum when the negation overflows?

-- 
Mark Mitchell
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