[RISCV] Check register class for AMO memory operands

Summary:
AMO memory operands use a custom parser in order to accept both (reg)
and 0(reg). However, the validation predicate used for these operands
was only checking that they were registers, and not the register class,
so non-GPRs (such as FPRs) were also accepted. Thus, fix this by making
the predicate check that they are GPRs.

Reviewers: asb, lenary

Reviewed By: asb, lenary

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72471