[llvm] [RISCV][GISel] Select G_FENCE. (PR #73184)

[Michael Maitland via llvm-commits]

https://github.com/michaelmaitland approved this pull request.

Do you mind adding some documentation for the G_FENCE opcode in another patch? Looks to be [missing](https://llvm.org/docs/GlobalISel/GenericOpcode.html#g-fence).

https://github.com/llvm/llvm-project/pull/73184