CSE SubAccesses (#2099) (#2102)

Fixes n^2 performance problem when dynamically indexing Vecs of
aggregate types.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
(cherry picked from commit e58ba0c1

)

Co-authored-by: Jack Koenig <koenig@sifive.com>