[OpenMP] Limit reduction support for pragma 'distribute' when combined with pragma 'simd'

Differential Revision: https://reviews.llvm.org/D43513

This is a bug fix that removes the emission of reduction support for pragma 'distribute' when found alone or in combinations without simd.
Pragma 'distribute' does not have a reduction clause, but when combined with pragma 'simd' we need to emit the support for simd's reduction clause as part of code generation for distribute. This guard is similar to the one used for reduction support earlier in the same code gen function.

llvm-svn: 325822