[unroll] Replace a linear time check for no uses with a constant time

[unroll] Replace a linear time check for no uses with a constant time
check.

Also hoist this into the enqueue process as it is faster even than
testing the worklist set, we should just directly filter these out much
like we filter out constants and such.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229056 91177308-0d34-0410-b5e6-96231b3b80d8