[unroll] Rather than an operand set, use a setvector for the worklist.

[unroll] Rather than an operand set, use a setvector for the worklist.

We don't just want to handle duplicate operands within an instruction,
but also duplicates across operands of different instructions. I should
have gone straight to this, but I had convinced myself that it wasn't
going to be necessary briefly. I've come to my senses after chatting
more with Nick, and am now happier here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229054 91177308-0d34-0410-b5e6-96231b3b80d8