[OpenMP] Avoid reading uninitialized parallel level values

In a last minute change request for a2dbfb6b we introduced a
read of the uninitialized parallel level value in SPMD-mode.
We go back to initializing the array early and checking for an
adjusted level.

Found by the miniqmc unit tests:
  https://cdash.qmcpack.org/CDash/viewTest.php?onlyfailed&buildid=203434

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D101123