Performance improvement: accessing thread struct as opposed to team struct

Replaced readings of nproc from team structure with ones from
thread structure to improve performance.

Patch by Andrey Churbanov.

Differential Revision: http://reviews.llvm.org/D21559


git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@273293 91177308-0d34-0410-b5e6-96231b3b80d8