reroll() was slightly monolithic and a pain to modify. Refactor
a bunch of its state from local variables to member variables
of a helper class, and do some trivial simplification while we're
there.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227439 91177308-0d34-0410-b5e6-96231b3b80d8