Fix dshl zero-width shift behavior (#2339)

* Fix dshl zero-width shift behavior

Add a special case for dshl handling in the ZeroWidths pass.  If one
expression is shifted by a second, zero-width expression, just return
the first expression.  This prevents a bug where the width will
incorrectly expand due to zero-widths introducing a 1-bit zero
expression.

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

* fixup! Fix dshl zero-width shift behavior

(cherry picked from commit cc80c631)