Improve RemoveReset handling of invalid inits

This modifies RemoveReset to NOT generate a mux for
invalid (IsInvalid) inits. In the case of an invalid init, the reset
is converted to a self-connect and no mux is generated.

This is implemented as a new, initial pass over the module to populate
a set of all invalid signals. During the subsequent, circuit-modifying
pass, this invalid set is queried to special case the handling of
invalid inits.

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