[ADT] Remove 0-width Asserts in APInt.getZExtValue

Remove assertion that disallows getting a zero-extended value from a
zero-width APInt.  This check is too restrictive and makes it difficult
to use APInt to model zero-width things, e.g., zero-width wires in the
CIRCT project.

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

Reviewed By: lattner, darthscsi, nikic

Differential Revision: https://reviews.llvm.org/D114768