RFR: 8240259: Disable -Wshift-negative-value warnings

[Kim Barrett]

> On May 28, 2020, at 8:23 PM, Ioi Lam <ioi.lam at oracle.com> wrote:
> 
> Looks good to me.

Thanks.

> When I enabled c++14 in gcc, I had to do a few of these manually and didn't see any test failures, so I think the gcc warning isn't really catching anything bad in our code.
> 
> Thanks
> - Ioi
> 
> Node* RShiftINode::Identity(PhaseGVN* phase) {
>    ...
> #ifdef __GNUC__
> #pragma GCC diagnostic push
> #pragma GCC diagnostic ignored "-Wshift-negative-value"
> #endif
>     int lo = (-1 << (BitsPerJavaInteger - ((uint)shift)-1)); // FFFF8000
> #ifdef __GNUC__
> #pragma GCC diagnostic pop
> #endif

Yes, that one looks familiar.  I thought there were a couple more, but maybe
they disappeared in other code changes.  I haven’t tried doing a C++14 build
without that warning disabled for quite a while.