[OpenMP] Improve default block count selection fow low block counts

If a combined loop has insufficient parallelism (= low trip count), we
might end up with too few teams/blocks. To counter that we can reduce
the number of threads per team we use. This patch implements a heuristic
and exposes a new environment variable to control the minimum of threads
to be employed in this case.

Issue reported by:
Felipe Cabarcas Jaramillo <cabarcas@udel.edu> (@fel-cab).

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D152014