Improve TeddyHelper.RightShift helpers for ARM

[MihaZupan]

@tannergooding mentioned on Discord that AdvSimd.ExtractVector128 is the comparable ARM instruction to Ssse3.AlignRight.
This lets us delete the logic we were using to emulate the behavior via shuffles.

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Issue Details

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@tannergooding mentioned on Discord that AdvSimd.ExtractVector128 is the comparable ARM instruction to Ssse3.AlignRight.
This lets us delete the logic we were using to emulate the behavior via shuffles.

Author:	MihaZupan
Assignees:	MihaZupan
Labels:	area-System.Buffers
Milestone:	9.0.0

[tannergooding]

It's worth noting that...

AlignRight is exposed as (left, right, mask) where left is the upper 128-bits and right is the lower 128-bits of the concatenated vector, therefore mask 1 picks right[1]-right[15] and left[0] as the last element.

AdvSimd.ExtractVector128 is exposed as (upper, lower, mask), but it should've been (lower, upper, mask), so its inverse from the AlignRight order. This can be potentially confusing when reviewing the code.

The PR here is doing it correctly, just wanted to call out that its a weird thing if you go look at stuff

[MihaZupan]

@MihuBot -arm