[Bug target/89213] Optimize V2DI shifts by a constant on power8 & above systems.

[meissner at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89213

--- Comment #3 from Michael Meissner <meissner at gcc dot gnu.org> ---
The vector shift sequence does not appear in Spec 2006 CPU compiled for power9.

The vector shift sequence does appear 4 times in the 602_gcc_s benchmark, and
once in the 683_imagick_s benchmark.