Move llvm::computeLoopSafetyInfo from LICM.cpp to LoopUtils.cpp. NFC

Move computeLoopSafetyInfo, defined in Transforms/Utils/LoopUtils.h,
into the corresponding LoopUtils.cpp, as opposed to LICM where it resides
at the moment. This will allow other functions from Transforms/Utils
to reference it.

llvm-svn: 325151