[llvm] [RISCV] Remove the offset numbers from the FixedCSRFIMap. NFC (PR #129297)

[Sam Elliott via llvm-commits]

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

LGTM for these registers.

We have similar code downstream for Xqciint register placement, which is a little more complex as there are gaps and some registers are out of order (for frame pointer reasons). I will work out if I can simplify that code as I upstream it.

https://github.com/llvm/llvm-project/pull/129297