Fix PPC64 CR spill location for callee-saved registers

This fixes an ABI bug for non-Darwin PPC64. For the callee-saved condition
registers, the spill location is specified relative to the stack pointer (SP +
8). However, this is not relative to the SP after the new stack frame is
established, but instead relative to the caller's stack pointer (it is stored
into the linkage area of the parent's stack frame).

So, like with the link register, we don't directly spill the CRs with other
callee-saved registers, but just mark them to be spilled during prologue
generation.

In practice, this reverts r179457 for PPC64 (but leaves it in place for PPC32).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179500 91177308-0d34-0410-b5e6-96231b3b80d8
Mark all PPC comparison instructions as not having side effects

Now that the CR spilling issues have been resolved, we can remove the
unmodeled-side-effect attributes from the comparison instructions (and also
mark them as isCompare). By allowing these, by default, to have unmodeled side
effects, we were hiding problems with CR spilling; but everything seems much
happier now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179502 91177308-0d34-0410-b5e6-96231b3b80d8