[Patch] sext_hwi: Avoid left shift of negative value undefined behaviour

[Jeff Law]

On 08/12/2015 11:12 AM, Richard Biener wrote:

>
> Prec is almost never a constant and is heavily used from wide-int.
>
> We are not exploiting this undefined ness in C so I object to making this so much slower.
>
> Can we instead do what we do for abs_hwi and add a checking assert so we can move the tests to the callers that misbehave instead?
Given that ISO C++ is moving away from making shifting 1 into the sign 
bit undefined behaviour, maybe we should make UBSan less strict in its 
warning.  That may eliminate the need for Mikael's patch.

jeff