The region within sparse_tensor.select is used as the runtime criteria
for whether to keep the existing value in the sparse tensor.
While the sparse element is provided to the comparison, indices may also
be used to decide on whether to keep the original value. This allows, for
example, to only keep the upper triangle of a matrix.
Reviewed by: aartbik
Differential Revision: https://reviews.llvm.org/D134761
