[OpenMP][FIX] Pass the num_threads value directly to parallel_51

The problem with the old scheme is that we would need to keep track of
the "next region" and reset the num_threads value after it. The new RT
doesn't do it and an assertion is triggered. The old RT doesn't do it
either, I haven't tested it but I assume a num_threads clause might
impact multiple parallel regions "accidentally". Further, in SPMD mode
num_threads was simply ignored, for some reason beyond me.

In any case, parallel_51 is designed to take the clause value directly,
so let's do that instead.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D113623