vkd3d-shader: Make ins_it actually optional in vsir_program_iterator_insert_before().

This patch is currently part of !1700 (closed), but probably it can be discussed separately.

As the comment says, vsir_program_iterator_insert_before() is intended to receive 2 iterator pointers. The second one is an output set to the first inserted instruction and it should be optional.

My original implementation in !1707 (merged) made it indeed optional but it got changed a bit before being upstreamed.

I am okay if we prefer it not being optional, but in that case we would have to modify a bit the comment.

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