[OpenACC][CIR] Implement 'num_gangs' lowering (#137216)
This is similar to the previous handful of lowering commits, except that
it takes an array of int-expressions rather than a single one. This
complicates the list of things that need updating (as the 'segments'
array also needs updating), which resulted in a bit of a refactor.
At the moment, only parallel/kernels are enabled (not parallel
loop/kernels loop), so tests are added just for those.
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Co-authored-by: Valentin Clement (バレンタイン クレメン) <clementval@gmail.com>
