add sv support for zeroing predication in dest register

add sv support for zeroing predication in dest register

bit of a major rework:

* access to the "unpredicated" (non-zero-hacked) register was needed
* therefore all rd/rs1-3/rvc_xxx functions had to have _ variants
* the underscored variants are not predicated
* this in turn meant that the offset for each register was wrong
  as it is incremented *after* being checked
* therefore a newoffs had to be added
* and the reset_cache function copies the newoffs values

bit of a mess but it works: this is a state machine after all...