x86: Optimize svml_s_tanhf8_core_avx2.S

x86: Optimize svml_s_tanhf8_core_avx2.S

Optimizations are:
    1. Reduce code size (-81 bytes).
    2. Remove redundant move instructions.
    3. Slightly improve instruction selection/scheduling where
       possible.
    4. Prefer registers which get short instruction encoding.
    5. Reduce rodata size (-32 bytes).

Result is roughly a 17-18% speedup:

       Function, New Time, Old Time, New / Old
_ZGVdN8v_tanhf,     1.977,    2.402,     0.823