This makes the structure of the vec4 live intervals calculation more
similar to the FS back-end liveness analysis code. The non-CF-aware
start/end computation is moved into the same pass that calculates the
block-local def/use sets, which saves quite a bit of code, while the
CF-aware start/end computation is moved into a separate
compute_start_end() function as is done in the FS back-end.
Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4012>
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