Lose some weight parameters in WebGL 4096, 8192 mode.

[Kiikurage]

This problem is originally reported in #816.

In WebGL backend, weight data is smaller than expected. This problem is occurred only when texture size is 4096 or 8192. This may be caused by graph splitting.

[Kiikurage]

This problem is occurred in follow sub graph,

x -{op1}-> y -{op2} -> z -

where x is ConstantVariable and y is splitting target. In some graph splitting rules, another optimization rule constant_folding is reused. But this rule modifies the original sub graph as follows,

y 
                  
y' -{op2} -> z -

where y' is ConstantVariable, which is the result of applying op1 to x. However, splitting target is not updated from y to y'. Therefore, follow subgraph are generated.

y1 -{op2_1}-> z1-+
                 +-{merge}-> z -
y2 -{op2_2}-> z2-+
                  
y' -{op2}

As a result, x's data is left at disconnected sub graph.