perf: speed up s8q NHWC max pooling

[morgolock]

Refactor the A64, SVE and SME QASYMM8_SIGNED differing-qinfo NHWC MAX pooling kernels to reduce the cost of the requantized path.

The old code did:

• add output offset
• explicit clamp to [-128, 127] with smax / smin
• pack down with several uzp1 shuffles

The new code does:

• add output offset
• saturating narrow directly with sqxtn / sqxtn2 from s32 -> s16 -> s8

Why this helps:

• sqxtn already performs the signed saturation, so the explicit clamp is redundant.
• It also lets us remove a chunk of shuffle/packing work.
• So the requantized epilogue gets shorter: fewer instructions, less register traffic, less packing overhead.

For the tested diff-qinfo cases this improves steady-state latency by:

• A64: 4.7% to 5.3%
• SVE2: 0.8% to 3.9%
• SME2: 3.9% to 8.6%

Change-Id: I47a378aeda61de86f2d6784393ccb4a08984706c

[gunes-arm]

Nice work Pablo. I only have doc-related suggestions.

(1)
I suggest we remove the following section from the commit description as it's highly input configuration dependent:

For the tested diff-qinfo cases this improves steady-state latency by:
- A64: 4.7% to 5.3%
- SVE2: 0.8% to 3.9%
- SME2: 3.9% to 8.6%

(2) MR title and commit title should be the same. I'd say the following is fine; note the capital start:
perf: Speed up s8q NHWC max pooling