[OpenMP] Reductions with non-trivial types (e.g., complex) cause SPMD-zation to fail

[jdoerfert]

SPMD-zation is required for good performance and the copy constructor used for non-trivial types can cause us to miss out on it.

While SPMD-zation has conceptual limitations right now, the cases I've seen with complex reductions are fairly simple. While investigating #52706 we noticed this performance bug.

[llvmbot]

@llvm/issue-subscribers-openmp

[tstellar]

This issue was not resolved in time for 14.0.0 and will need to be fixed in the next release (14.0.1). If you disagree with this or you feel there was a mistake, please change the milestone back to 'LLVM 14.0.0 Release' so we can discuss.