HLSL: Partially flatten hierarchies, instead of all or nothing.

Fixes #1092.  Allows arrays of opaques to keep arrayness, unless
needed by uniform array flattening.
Can handle assignments of mixed amounts of flattening.

Test/hlsl.partialFlattenLocal.vert

@@ -0,0 +1,27 @@
+Texture2D tex;
+
+struct Packed {
+  Texture2D     tex;
+  float3        pos[3];
+  float2        uv[2];
+  float         x;
+  int           n;
+};
+
+float4 main(float4 pos : POSITION) : SV_POSITION
+{
+  Packed packed;
+  packed.tex    = tex;
+  packed.pos[0] = float3(0, 0, 0);
+  packed.uv[0]  = float2(0, 1);
+  packed.x      = 1.0;
+  packed.n      = 3;
+
+  for (int i = 0; i < 1; ++i) {
+    packed.pos[i].xy += packed.uv[i];
+  }
+
+  Packed packed2 = packed;
+
+  return pos + float4(packed2.pos[0], 0);
+}