[PM] Simplify the new PM interface to the loop unroller and expose two

[PM] Simplify the new PM interface to the loop unroller and expose two
factory functions for the two modes the loop unroller is actually used
in in-tree: simplified full-unrolling and the entire thing including
partial unrolling.

I've also wired these up to nice names so you can express both of these
being in a pipeline easily. This is a precursor to actually enabling
these parts of the O2 pipeline.

Differential Revision: https://reviews.llvm.org/D28897

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@293136 91177308-0d34-0410-b5e6-96231b3b80d8