[LoopReroll] Alter the data structures used during reroll validation.

The validation algorithm used an incremental approach, building each
iteration's data structures temporarily, validating them, then
adding them to a global set.

This does not scale well to having multiple sets of Root nodes, as the
set of instructions used in each iteration is the union over all
the root nodes. Therefore, refactor the logic to create a single, simple
container to which later logic then refers. This makes it simpler
control-flow wise to make the creation of the container more complex with
the addition of multiple root sets.

llvm-svn: 227499