Speed up ExpandWhens for very large designs (#1666) (#1674)

Use WrappedExpression instead of MemoizedHash. The benefit of memoizing
the hash pales in comparison to the cost of hashing deeply nested Types
in the AST.

Co-authored-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>
(cherry picked from commit bffc340e

)

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