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 #101770 for history.

[dotnet-policy-service]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

[kunalspathak]

#104640 falls in similar category.

[a74nh]

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