Fix &A<> to support an undefined word index.
Fix &A<> to support an undefined word index.

This patch adds support to &A<> for an undefined word index.
It mostly matches what was done for &PV<>.

CastMi/iverilog,CastMi/iverilog,themperek/iverilog,themperek/iverilog,CastMi/iverilog,themperek/iverilog