[FIRRTL] Update NLA during prefix module pass

[prithayan]

This change updates the NonLocalAnchor symbol reference to modules that are renamed in prefix module pass.
The nla op contains reference to module symbols, which might be left invalid after module renaming.
firrtl.nla @nla_2 [@Top, @Aardvark, @Zebra] ["test", "test", "Zebra"]

There can be multiple copies of any module created after the prefix module pass.
The firrtl.nla renaming is context sensitive and depends on the instance op, which is anchored to the nla.

This renaming is also based on the assumption, that the modules in the firrtl.nla begin with the top level module.
Otherwise, the firrtl.nla might be ambiguous if multiple clones of the first module is created.

This change is required for OMIR emission, since prefix module pass leaves the tracker firrtl.nla in invalid state.

[seldridge]

Awesome. Thanks for taking care of this, Prithayan!

[prithayan]

Thanks @seldridge , Fixed the loop to iterate once over the annotations.