This revision introduces a pattern to lower `gpu.subgroup_reduce` op into to the `nvvm.redux_sync` op. The op must be run by the entire subgroup, otherwise it is undefined behaviour.
It also adds a flag and populate function, because the op is not avaiable for every gpu (sm80+), so it can be used when it is desired.
Depends on D142088
Reviewed By: nicolasvasilache
Differential Revision: https://reviews.llvm.org/D142103
a3388f3e2a