RegisterClassInfo: Fix CSR cache invalidation

`RegisterClassInfo` caches information like allocation orders and reuses
it for multiple machine functions where possible. However the `MCPhysReg
*CalleeSavedRegs` field used to test whether the set of callee saved
registers changed did not work: After D28566
`MachineRegisterInfo::getCalleeSavedRegs()` can return dynamically
computed CSR sets that are only valid while the `MachineRegisterInfo`
object of the current function exists.

This changes the code to make a copy of the CSR list instead of keeping
a possibly invalid pointer around.

Differential Revision: https://reviews.llvm.org/D132080