Move definitions of lopoUnroll* functions to LoopUtils.cpp.

These functions are declared in Transforms/LoopUtils.h (included to the
Transforms/Utils library) but were defined in the loop unrolling pass in
Transforms/LoopUnroll.cpp.  As a result, targets depending only on
TransformUtils library but not on Transforms could get link errors.  Move the
definitions to Transforms/Utils/LoopUtils.cpp where they should actually live.
This does not modify any code.

PiperOrigin-RevId: 221508882