[RISCV] Fix regression due to interaction of MachineOutliner and MachineCopyPropagation

D144535 enabled machine copy propagation for RISC-V and added it to the
pass pipeline in addPreEmitPass2 (after the MachineOutliner).
Unfortunately, the MachineCopyPropagation pass is unable to correctly
analyse outlined functions, and will delete copy instructions where a
register is set that is intended to be live-out.
RISCVInstrInfo::buildOutlinedFrame will directly insert a JALR, while a
similar function going through the normal codegen path would have a
PseudoRet with operands indicating registers that are live-out.

This patch does the simplest fix, which is to run MachineCopyPropagation
before the MachineOutliner.

Differential Revision: https://reviews.llvm.org/D146037