llvm dev - Mar 2018 - [NFC] Restructuring LoopRoration.cpp to create Loop Rotation Pass with Loop Rotation Utility Interface

NFC: Restructuring LoopRoration.cpp to create Loop Rotation Pass with Loop
Rotation Utility Interface
============================================================================
Why this NFC change?
================The existing LoopRotation.cpp is implemented as one of loop
passes instead of being a utility. The user cannot easily perform the loop
rotation selectively (or on demand) under different optimization level. For
example, the loop rotation is needed as part of the logic to convert a loop into
a loop with bottom test for a transformation. If the loop rotation is simply
added as a loop pass before the transformation, the pass is skipped if it is
compiled at –O0 or if it is explicitly disabled by the user, causing the
compiler to generate incorrect code. Furthermore, as a loop pass it will rotate
all loops instead of just the relevant loops.


Change Made
=========We provide a utility interface for the loop rotation so that the loop
rotation can be called on demand.  The changeset is as follows:
1)  Create a new file lib/Transforms/Utils/LoopRotationUtils.cpp and move the
main implementation of class LoopRotate into this file.
2)  Create a new file llvm/include/Transform/Utils/LoopRotationUtils.h with the
interface LoopRotation(…).
3)  Original LoopRotation.cpp is changed to use the utility function
LoopRotation in LoopRotationUtils.cpp. This is done in the same way community
did for mem-to-reg implementation.

=======Summary:
=======Provide an interface for loop rotation so that it can be called on
demand. The change is simple and straightforward, patch is uploaded with this
NFC for code review.

https://reviews.llvm.org/D44595

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