Adds a virtual method, GetFreshIds(), to Transformation. Every
transformation uses this to indicate which ids in its protobuf message
are fresh ids. This means that when replaying a sequence of
transformations the replayer can obtain a smallest id that is not in
use by the module already and that will not be used by any
transformation by necessity. Ids greater than or equal to this id
can be used as overflow ids.
Fixes#3851.
This PR introduces TransformationAddLoopPreheader, which, given
a loop header and enough fresh ids, adds a loop preheader, updating
all the references so that this new block is the only out-of-loop
predecessor of the header, which branches unconditionally to the
header.
See the discussion in #3095.
