[RISCV] Support named operands for CSR instructions.

Reviewers: asb, mgrang

Reviewed By: asb

Subscribers: jocewei, mgorny, jfb, PkmX, MartinMosbeck, brucehoult, the_o, rkruppe, rogfer01, rbar, johnrusso, simoncook, jordy.potman.lists, sabuasal, niosHD, kito-cheng, shiva0217, zzheng, edward-jones

Differential Revision: https://reviews.llvm.org/D46759

llvm-svn: 343822