[llvm] [RISCV][MC] Recognise that fcvt.d.s with frm != 0b000 is valid (PR #67555)

[Alex Bradbury via llvm-commits]

asb wrote:

I've edited the original patch description to clarify this, but wanted to add a comment as well for visibility. I had at one time been concerned about the `rne` rounding mode being printed by us and being rejected by GNU as, but that's not actually an issue provided the InstAlias without the rounding mode is used. This does work for rv32d/rv64d but _doesn't_ seem to be working properly for `dinx` for reasons I haven't yet investigated. We also clearly want test coverage for this as well, given the printing or not of `rne` is important.

Apologies for the confusion on that point. A quick review in terms of the general approach (and to ensure you agree there is a bug to be fixed) would be helpful, but this isn't ready for a detailed review until I add test coverage for the rounding mode printing and resolve whatever the issue with the InstAlias is on dinx.

https://github.com/llvm/llvm-project/pull/67555