Refine the loop rotation's API

Summary:
The following changes addresses the following two issues.

1) The existing loop rotation pass contains both loop latch simplification and loop rotation. So one flag RotationOnly is added to be passed to the loop rotation pass.
2) The threshold value is initialized with MAX_UINT since the loop rotation utility should not have threshold limit.

Reviewers: dmgreen, efriedma

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D45582

llvm-svn: 330362