When a triplet's lower and upper bounds are the same, we can
check them without requiring a constant stride.
Differential Revision: https://reviews.llvm.org/D142943
Semantics can catch out-of-range subscript values already when they
appear in DATA statement objects and constant folding of name constant
array indexing; this patch fills the gap by checking known constant
subscript values in other contexts.
Differential Revision: https://reviews.llvm.org/D142764