[RISCV] Make CanLowerReturn protected for downstream maintenance

Summary: For the downstream RISCV maintenance, it would be easier to override and reuse CanLowerReturn for customizing.

Reviewers: asb, lenary, luismarques

Reviewed By: lenary

Subscribers: hiraditya, rbar, johnrusso, simoncook, sabuasal, niosHD, kito-cheng, shiva0217, jrtc27, MaskRay, zzheng, edward-jones, rogfer01, MartinMosbeck, brucehoult, the_o, rkruppe, PkmX, jocewei, psnobl, benna, s.egerton, pzheng, sameer.abuasal, apazos, evandro, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78545