HLSL: add .mips[][] operator for texture types

This implements mytex.mips[mip][coord] for texture types.  There is
some error testing, but not comprehensive.  The constructs can be
nested, e.g in this case the inner .mips is parsed before the completion
of the outer [][] operator.

   tx.mips[tx.mips[a][b].x][c]

Test/hlsl.mip.operator.frag

@@ -0,0 +1,14 @@
+Texture2DArray     g_tTex2df4a;
+Texture2D          g_tTex2df4;
+
+float4 main() : SV_Target0
+{
+    return g_tTex2df4.mips[2][uint2(3, 4)] +
+
+        // test float->uint cast on the mip arg
+        g_tTex2df4a.mips[5.2][uint3(6, 7, 8)] +
+
+        // Test nesting involving .mips operators:
+        //               ....outer operator mip level......     .....outer operator coordinate....
+        g_tTex2df4.mips[ g_tTex2df4.mips[9][uint2(10,11)][0] ][ g_tTex2df4.mips[13][uint2(14,15)].xy ];
+}