Use inline Vector{128|256}.Create for constants

[gfoidl]

Fixes #44115

Used Vector(128|256)<\w+> \w+ = as regex-search, seems that these three places are the onley one left.
The SSE-helper, that are mentioned in the issue, got re-written by @GrabYourPitchforks some time ago, and there patterns for the new codegen is already used.

Codegen was validated (manually) to see that it's an improvement.

It's not clear to me whether the touched codes are built for .NET Core 3.1 or not. So I added conditional compilation, as for .NET Core 3.1 the codegen is quite worse when Vector{128|256}.Create is used (that's why we initially had the different patterns before the JIT started to emit constants and read them from the data-segment).

/cc: @tannergooding

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[stephentoub]

Why are these ifdefs needed... isn't this file only compiled for netcoreappcurrent?

[gfoidl]

isn't this file only compiled for netcoreappcurrent?

This is the point I don't know (alluded in the description).
But if so, I'll just remove them.

My point is just to make sure that .NET Core 3.1 doesn't regress, as it's a LTS through Dec, 2022.

[stephentoub]

They're not needed. This file is only included in System.Collections.csproj, which only builds for netcoreappcurrent:

runtime/src/libraries/System.Collections/src/System.Collections.csproj

Line 5 in bde0322

<TargetFrameworks>$(NetCoreAppCurrent)</TargetFrameworks>

Base64Decoder/Encoder.cs are only included in System.Memory, which only builds for netcoreappcurrent:

runtime/src/libraries/System.Memory/src/System.Memory.csproj

Line 6 in bde0322

<TargetFrameworks>$(NetCoreAppCurrent)</TargetFrameworks>

[gfoidl]

Thanks! 👍🏻

[ghost]

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