Add constant prop to async regs (bp #1355) (#1363)

* 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 952bf381.

(cherry picked from commit b36e36d9

)

* Add unknowntype to Mux as applys werent backported

Co-authored-by: Adam Izraelevitz <adam.izraelevitz@sifive.com>