Previously each time we saw a variable we just created a duplicate
entry in the list. This is particularly bad for loops were we add
everything twice, and then throw nested loops into the mix and the
list was growing expoentially.

This stops the glsl-vs-unroll-explosion test which has 16 nested
loops from reaching the tests mem usage limit in this pass. The
test now hits the mem limit in opt_copy_propagation_elements()
instead.

I suspect this was also part of the reason this pass can be so
slow with some shaders.

Reviewed-by: Thomas Helland <thomashelland90@gmail.com>