Revert "Revert the Metal Experiment (#701)"

This reverts commit fe1617ffea.

src/Ryujinx.Graphics.Shader/CodeGen/Msl/HelperFunctions/SwizzleAdd.metal

@@ -0,0 +1,7 @@
+float swizzleAdd(float x, float y, int mask, uint thread_index_in_simdgroup)
+{
+    float4 xLut = float4(1.0, -1.0, 1.0, 0.0);
+    float4 yLut = float4(1.0, 1.0, -1.0, 1.0);
+    int lutIdx = (mask >> (int(thread_index_in_simdgroup & 3u) * 2)) & 3;
+    return x * xLut[lutIdx] + y * yLut[lutIdx];
+}