[RISCV][NFC] Use NoRegister instead of 0 literal

[RISCV][NFC] Use NoRegister instead of 0 literal

Summary: Trivial cleanup.

Reviewers: asb, lenary

Reviewed By: lenary

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67526

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@372120 91177308-0d34-0410-b5e6-96231b3b80d8