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

[Alex Bradbury via llvm-commits]

asb wrote:

For convenience, here is a relevant snippet from the ISA manual. fcvt.d.s will never round, but the frm field isn't listed as being fixed to 0b000.
![fcvt](https://github.com/llvm/llvm-project/assets/1900/9097c5ed-0a59-41c4-adec-affe9e9b95b2)


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