[OpenMP] Remove optimization skipping reduction struct initialization (#65697)

This commit removes an optimization that skips the initialization of the
reduction struct if the number of threads in a team is 1. This
optimization
caused a bug with Hidden Helper Threads. When the task group is
initially
initialized by the master thread but a Hidden Helper Thread executes a
target
nowait region, it requires the reduction struct initialization to
properly
accumulate the data.

This commit also adds a LIT test for issue #57522 to ensure that the
issue is
properly addressed and that the optimization removal does not introduce
any
regressions.

Fixes: #57522