Propagate source locators to register update always blocks (#1743)

* [WIP] Propagate source locators to Verilog if-else emission

* Add and fix tests for reg update info propagation

* Add limited source locator propagation in ConstProp

Support propagating source locators on connections or nodes where the
right-hand side is simply a reference. This case comes up a lot for
registers without a synchronous reset.

node _T_1 = x @[MyFile.scala 12:10]
node _T_2 = _T_1
z <= x

Previousy the source locator would be lost, now the result is:

z <= x @[MyFile.scala 12:10]

* Address review comments

Co-authored-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

Co-authored-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>