[lldb] Remove some unreachable code (NFC) (#190529)
`isRISCV()` check always returns false because we only get here if
`min_op_byte_size` and `max_op_byte_size` are equal, which is not true
for RISC-V.
Also, replase `if (!got_op)` check with an `else`. The check is
equivalent to
`if (min_op_byte_size != max_op_byte_size)`, and the `if` above checks
for the opposite condition.
