Add recursive task scheduling strategy to taskloop implementation

Summary:
Taskloop implementation is extended by using recursive task scheduling.
Envirable KMP_TASKLOOP_MIN_TASKS added as a manual threshold for the user
to switch from recursive to linear tasks scheduling.

Details:
* The calculations for the loop parameters are moved from __kmp_taskloop_linear
  upper level
* Initial calculation is done in the __kmpc_taskloop, further range splitting
  is done in the __kmp_taskloop_recur.
* Added threshold to switch from recursive to linear tasks scheduling;
* One half of split range is scheduled as an internal task which just moves
  sub-range parameters to the stealing thread that continues recursive
  scheduling (if number of tasks still enough), the other half is processed
  recursively;
* Internal task duplication routine fixed to assign parent task, that was not
  needed when all tasks were scheduled by same thread, but is needed now.

Patch by Andrey Churbanov

Differential Revision: https://reviews.llvm.org/D35273

llvm-svn: 308338