CHERI-RISC-V: slightly rearrange SLTI HINT decoder

- Restructure the code under the assumption that there will be HINT consumers
  other than TCG_LOG_INSTR.

- Document the current and near-term allocations of the opcode space:

  - Move the TCG_LOG_INSTR tests to match rs1 == x0, which is the only form that
    we have emitted, but we were never ignoring rs1.

  - Reserve `SLTI x0, x31, 0xNNN` forms for software's introspective use until
    (and if) RISC-V upstream defines something for that use case.