[Clang][OpenMP] Monotonic does not apply to SIMD

The codegen for simd constructs was affected by the presence (or
absence) of the 'monotonic' schedule modifier for worksharing
loops. The modifier is only intended to apply to the scheduling of
chunks for a thread, not iterations of a loop inside a chunk.

In addition, the monotonic modifier was applied to worksharing loops
by default if no schedule clause was present; the referenced part of
the OpenMP 4.5 spec in the code (section 2.7.1) only applies if the
user specified a schedule clause with a static kind but no modifier.
Without a user-specified schedule clause we should default to
nonmonotonic scheduling.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D103793