bf16 matmul's corresponding `tensor.pack` not properly optimized

Currently, the following 2 single-layer MLP have worst performance compared with GC v1.

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dtype | batch size | hidden list | GC V1 | 8c55a0544 remove brgemm read lock
-- | -- | -- | -- | --
bf16 | 128 | 1024x1024 | 0.0286 | 0.0828 | 34.52%
bf16 | 128 | 1024x512 | 0.0204 | 0.0670 | 30.45%

We performed detailed breakdown as follows:

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128x1024x1024 | GC v1 | 8c55a0544
-- | -- | --
matmul only | 0.01766 | 0.01989
tiled pack (or reorder) | 0.02634 | 0.04632
total | 0.04418 | 0.077969

and

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128x1024x512 | GC v1 | 8c55a0544
-- | -- | --
matmul only | 0.01587 | 0.01591
tiled pack (or reorder) | 0.01278 | 0.0398
total | 0.02881 | 0.06917

Are there any further optimization opportunity for vnni pack?