[flang] Clarify CheckReductionDIM()

This utility routine in constant folding should return false when
a DIM= actual argument to a reduction intrinsic function has a
value that prevents folding, and true when folding can proceed.
The implementation was returning true in cases where a DIM=
argument was present but not constant.

Clarify the code and add commentary: a true result means that
there is no DIM= actual argument present, or that a DIM= argument
exists, is constant, and has a value that is in range.

Differential Revision: https://reviews.llvm.org/D131101