x86: Improve svml_s_atanhf8_core_avx2.S

x86: Improve svml_s_atanhf8_core_avx2.S

Improvements are:
    1. Reduce code size (-60 bytes).
    2. Remove redundant move instructions.
    3. Slightly improve instruction selection/scheduling where
       possible.
    4. Prefer registers which get short instruction encoding.
    5. Shrink rodata usage (-32 bytes).

The throughput improvement is not that significant (3-5%) as the
port 0 bottleneck is unavoidable.

       Function, New Time, Old Time, New / Old
_ZGVdN8v_atanhf,    2.799,    2.923,     0.958