These track the same information in a slightly different way. Since
nir_loop_variable::update_src is visible outside this module, it cannot
be eliminated.
This leads to some nice simplification in find_trip_count. Previously
this code only had access to the ALU instruction that performs the
increment. It had to "search" the parameters to determine which (if any)
was the constant. With this change, this code has access to the
nir_alu_src of the ALU instruction that performs the increment. It no
longer needs to search the parameters for the constant. It's either the
supplied nir_alu_src or nothing.
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21289>
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