[OpenMP] Avoid VLAs for some reductions on array sections

In some cases the compiler can deduce the length of an array section
as constants. With this information, VLAs can be avoided in place of
a constant sized array or even a scalar value if the length is 1.
Example:
int a[4], b[2];
pragma omp parallel reduction(+: a[1:2], b[1:1])
{ }

For chained array sections, this optimization is restricted to cases
where all array sections except the last have a constant length 1.
This trivially guarantees that there are no holes in the memory region
that needs to be privatized.
Example:
int c[3][4];
pragma omp parallel reduction(+: c[1:1][1:2])
{ }

Differential Revision: https://reviews.llvm.org/D39136

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@316229 91177308-0d34-0410-b5e6-96231b3b80d8