[AArch64][SVE] Use FeatureUseFixedOverScalableIfEqualCost for A320

[stuij]

Downstream issue: #482

With this new A320 in-order core, we follow adding the FeatureUseFixedOverScalableIfEqualCost feature to A510 and A520 (#132246), which reaps the same code generation benefits of preferring fixed over scalable when the cost is equal.

So when we have:

void foo(float* a, float* b, float* dst, unsigned n) {
    for (unsigned i = 0; i < n; ++i)
        dst[i] = a[i] + b[i];
}

When compiling without the feature enabled, we get:

...
    ld1b    { z0.b }, p0/z, [x0, x10]
    ld1b    { z2.b }, p0/z, [x1, x10]
    add     x12, x0, x10
    ldr     z1, [x12, #1, mul vl]
    add     x12, x1, x10
    ldr     z3, [x12, #1, mul vl]
    fadd    z0.s, z2.s, z0.s
    add     x12, x2, x10
    fadd    z1.s, z3.s, z1.s
    dech    x11
    st1b    { z0.b }, p0, [x2, x10]
    incb    x10, all, mul #2
    str     z1, [x12, #1, mul vl]
...

When compiling with, we get:

...
  	ldp	    q0, q1, [x12, #-16]
	ldp	    q2, q3, [x11, #-16]
	subs	x13, x13, #8
	fadd	v0.4s, v2.4s, v0.4s
	fadd	v1.4s, v3.4s, v1.4s
	add	    x11, x11, #32
	add	    x12, x12, #32
	stp	    q0, q1, [x10, #-16]
	add	    x10, x10, #32

...

This patch also moves FeatureUseFixedOverScalableIfEqualCost for A510 and A520 from the CPU features to the tune features.

[github-actions]

This pull review modifies files outside of the arm-software directory, so please ensure it follows the Downstream Patch Policy.
An automated check will test if the tagging requirements have been met. Please wait for approving reviews from both Arm Toolchain for Embedded and Arm Toolchain for Linux teams before merging.

[stuij]

This is a cherry-pick from upstream head. I'm assuming we don't need to adhere to the Requirements for a downstream patch paragraphs in the contributing guide?

[davemgreen]

This looks the same as the upstream version, and should be pretty safe as it only alters tuning features. LGTM.

[MarkMurrayARM]

Looks good, and we have DavidG's reassurance. You may bypass the rules and merge this (Release manager hat on). MarkM

[dcandler]

This is a cherry-pick from upstream head. I'm assuming we don't need to adhere to the Requirements for a downstream patch paragraphs in the contributing guide?

On our branch, it's still a downstream change that needs tracking.

Also, a reminder:

Please wait for approving reviews from both Arm Toolchain for Embedded and Arm Toolchain for Linux teams before merging.

[tblah]

I think this is okay for ATfL as this is an upstream backport and doesn't impact the cores we benchmark against.

[stuij]

On our branch, it's still a downstream change that needs tracking.

Ok, thanks for the clarification. Done.

[dcandler]

LGTM, the only changes since the previous commit are additional comments which are non-functional so I think the previous approvals should still apply.