[release/5.0] Fix logic to test bits in a constant vector (Vector256)

[EgorBo]

Issue: #47385
Fix in master: #47385

Customer Impact

If a user tries to create a Vector256 via Create overload and the first half of its arguments are zeros and the second part is, let's say, all "1" (or any other constant) - a special optimization will always optimize that Create to Vector256<T>.Zero. A simple example:

Console.WriteLine(Vector256.Create(0, 0, 0, 0, 1, 1, 1, 1));

prints:

<0, 0, 0, 0, 0, 0, 0, 0>

but is expected to print:

<0, 0, 0, 0, 1, 1, 1, 1>

Same for -1 and AllBitSets, and same for Vector64 on arm64.
Where T type is pretty much any type Vectors support.

Testing

This PR adds a regression test.

Risk

Low. This impacts only rare specific cases with hardware intrinsics.

Regression

Yes, the optimization Vector256(0,0,0...0) to Vector256<>.Zero was introduced in 5.0

[echesakov]

LGTM

[JulieLeeMSFT]

CC @jeffschwMSFT

[jeffschwMSFT]

Approved. We should consider this for 5.0.x