Move PPCFrameLowering into PPCSubtarget from PPCTargetMachine. Use
the initializeSubtargetDependencies code to obtain an initialized
subtarget and migrate a couple of subtarget using functions to the
.cpp file to avoid circular includes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210822 91177308-0d34-0410-b5e6-96231b3b80d8