metal : Q3_K speedup (#2995)

* Slightly faster Q3_K and Q5_K on metal

* Another Q3_K speedup on metal

Combined with previous commit, we are now +9.6% for TG.
PP is not affected as this happens via the matrix multiplication
templates.

* Slowly progressing on Q3_K on metal

We are now 13% faster than master

* nother small improvement for Q3_K on metal

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Co-authored-by: Iwan Kawrakow <iwan.kawrakow@gmail.com>