ARM64-SVE: Detect mask usage for Across functions

[a74nh]

ConditionalSelect is used to mask an API. For methods that reduce down to a scalar value (eg all the Across methods), this would be done via:

a = Sve.AddAcross(Sve.ConditionalSelect(mask, a, zero));

Currently this will produce an SEL instruction to select the correct entries. Then will feed this into an SADDV (or UADDV) that uses an all-true mask.

Instead, the SEL needs merging into the SADDV to produce a single SADDV instruction using the mask.

See https://github.com/dotnet/runtime/issues/101770 for history.

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[kunalspathak]

https://github.com/dotnet/runtime/pull/104640 falls in similar category.

[a74nh]

priority:3 for RC1 snap : Performance issue. Produces code that is not ideal, but will not break anything.

[a74nh]

Given the backporting that would be required, I think it's too risky for NET9