[RISCV] Implement MC layer support for the fence.tso instruction

The instruction makes use of a previously ignored field in the fence
instruction. It is introduced in the version 2.3 draft of the RISC-V
specification after much work by the Memory Model Task Group.

As clarified here <https://github.com/riscv/riscv-isa-manual/issues/186>,
the fence.tso assembler mnemonic does not have operands.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@334278 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed