Add constant prop to async regs (#1355)

* Add constant prop to async regs

* Added another test of no reset value but constant assignment

* Clarify name of updateNodeMap

* Update constant assignment of async reset to not be inferred as a latch, works with donttouch

* Revert "Update constant assignment of async reset to not be inferred as a latch, works with donttouch"

This reverts commit 952bf38127cb32f814496a2b4b3bfb173d532728.