[PATCH]Clang and AArch64 backend patches to support sshll/ushll instructions

[Tim Northover]

Hi Jiangning,

> Should this be changed to
>
> #define vshll_high_n_s8(a, __b) __extension__ ({ \
>   int64x2_t __a = (a); \
>   (int16x8_t)vshll_n_s8(vget_high_s8(__a), __b); })

That sounds like a really good idea. Definitely an improvement on the
lots of repetitious C++ code we're going to end up with at the moment.

Tim.