[unroll] Don't check the loop set for whether an instruction is

[unroll] Don't check the loop set for whether an instruction is
contained in it each time we try to add it to the worklist, just check
this when pulling it off the worklist. That way we do it at most once
per instruction with the cost of the worklist set we would need to pay
anyways.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229060 91177308-0d34-0410-b5e6-96231b3b80d8