Similar to 'worker', the 'vector' clause has some rules that needed to
be applied on its argument legality that for combined constructs need to
look at the current construct, not the 'effective' parent construct.
Additionally, it has some interaction with `vector_length` that needed
to be encoded as well. This patch implements it.
OpenACC restricts the contents of a 'for' loop affected by a 'loop'
construct without a 'seq'. The loop variable must be integer, pointer,
or random-access-iterator, it must monotonically increase/decrease, and
the trip count must be computable at runtime before the function.
This patch tries to implement some of these limitations to the best of
our ability, though it causes us to be perhaps overly restrictive at the
moment. I expect we'll revisit some of these rules/add additional
supported forms of loop-variable and 'monotonically increasing' here,
but the currently enforced rules are heavily inspired by the OMP
implementation here.
The 'vector' clause specifies the iterations to be executed in vector or
SIMD mode. There are some limitations on which associated compute
contexts may be associated with this and have arguments, but otherwise
this is a fairly unrestricted clause.
It DOES have region limits like 'gang' and 'worker'.
