[OPENMP] Fixed codegen for 'reduction' clause.

Fixed codegen for reduction operations min, max, && and ||. Codegen for them is quite similar and I was confused by this similarity.
Also added a call to kmpc_end_reduce() in atomic part of reduction codegen (call to kmpc_end_reduce_nowait() is not required).
Differential Revision: http://reviews.llvm.org/D9513

llvm-svn: 236689