nir: Add an optional expression controlling nir_algebraic xforms.

nir: Add an optional expression controlling nir_algebraic xforms.

This will be used so that we can customize the transforms for the target
GPU, so we don't un-lower expressions that had already been lowered (or
introduce new lowering transformations that not all GPUs want)

v2: Drop the complication of having the condition->index dictionary, since
    we don't actually expect there to be many different conditions (change
    by Kenneth).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>