The RISC-V Assembly Programmer's Manual defines fp as another alias of x8. However, our tablegen rules only recognise s0. This patch adds fp as another alias of x8. GCC also accepts fp.
Thanks, I'd missed the fp alias as I was going by the riscv-elf-psabi-doc primarily.
I think rvi-alternate-abi-names.s would be a better name for the test. You can then add a comment explaining that: fp is an alternate ABI name for s0 it should be accepted in input, but s0 should be printed in preference. This is of course the behaviour your patch demonstrates!