fix Matrix4x4 + and - operator bugs

[saucecontrol]

The new SSE versions of the operators add/subtract value1 to/from itself. The tests were set up with the same value for both arguments, so they didn't catch the issue. Fixed both.

[danmoseley]

@tannergooding

[stephentoub]

This is a big functional regression, introduced in #31779 for 3.0 (cc: @EgorBo). This needs to be fixed for 3.0.

Example:

using System;
using System.Numerics;

class Program
{
    static void Main()
    {
        var m0 = new Matrix4x4(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0);
        var m1 = new Matrix4x4(1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1);
        Console.WriteLine(m0 + m1);
    }
}

On netcoreapp2.1:

{ {M11:1 M12:1 M13:1 M14:1} {M21:1 M22:1 M23:1 M24:1} {M31:1 M32:1 M33:1 M34:1} {M41:1 M42:1 M43:1 M44:1} }

On netcoreapp3.0:

{ {M11:0 M12:0 M13:0 M14:0} {M21:0 M22:0 M23:0 M24:0} {M31:0 M32:0 M33:0 M34:0} {M41:0 M42:0 M43:0 M44:0} }

[tannergooding]

@danmosemsft, do you want me to backport to release/3.0?

[danmoseley]

Yes, it sounds like it deserves it. Please include the usual tempalte in the PR https://gist.github.com/danmosemsft/348cbe83ccd7ae0e87c5598223be1380 and tag ask-mode.

[karelz]

This PR went into master - fixing milestone to 5.0.
It will be ported into release/3.0 branch in PR #39889.