[x86] Skylake tuning options

[Koval, Julia]

Hi,

This patch adds 2 tuning options for -mtune=skylake-avx512. Ok for trunk?

gcc/
        * x86-tune.def (movx, partial_reg_dependency): Enable for 
m_SKYLAKE_AVX512.

Thanks,
Julia

0001-ptc.patch
Description: 0001-ptc.patch