A little hacking in current generator allowed to reuse almost all code. That's nice!

Seems we'll need to refactor generator not to get lost in an if-maze.

After spilling to the stack all execute-related values,
we're still running out of registers. Need to figure out
which values from decode can be spilled too.

However, I'm unhappy with this, as my initial version of
decodeSync didn't use any stack. I believe it can be done,
but I'd like to share as many code as possible. On the other
hand, having code full of short if-statements is not readable.

I reclaimed three registers from adjustOffsets method, by reintroducing
its old version that didn't cache values in registers.

There are more and more if statements...

go test -run TestDecoder pass, multiframe tests still fail.