Fix a problem where the PPC backend lost track of the fact that it had

Fix a problem where the PPC backend lost track of the fact that it had
to save and restore the LR register on entry and exit of a leaf function
that needed to access globals or the constant pool.  This should hopefully
fix oscar from sending the PPC tester spinning out of control.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@20197 91177308-0d34-0410-b5e6-96231b3b80d8