Fix PR14364.

This removes a const_cast hack from PPCRegisterInfo::hasReservedSpillSlot().
The proper place to save the frame index for the CR spill slot is in the
PPCFunctionInfo object, not the PPCRegisterInfo object.

No new test cases, as this just reimplements existing function.  Existing
tests such as test/CodeGen/PowerPC/crsave.ll are sufficient.