nir: Move the if condition to a separate jump_if instruction

Now, every if statement will have a jump_if instruction in the block
before it that gives the condition. This removes a number of special
cases all over the place, and removes two pointers from nir_ssa_def,
which is a very memory-intensive data structure.

Unfortunately, nearly everything has to be updated at once, although
most of the changes are trivial.
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