Suport ir.SubAccess in Utils.splitRef (#2021)

* Add SubAccess case to Utils.splitRef

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

* Update Utils.splitRef to use IR types

Change Utils.splitRef to use the actual IR types instead of their
WIR aliases. Update the Scaladoc note to reflect this.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

Co-authored-by: Jack Koenig <koenig@sifive.com>
(cherry picked from commit 81ff0fe3)

# Conflicts:
#	src/main/scala/firrtl/Utils.scala