RISC-V: fix build after "Add support for arbitrary immediate encoding formats"

RISC-V: fix build after "Add support for arbitrary immediate encoding formats"

Pre- and post-increment/decrement are side effects, the behavior of
which is undefined when combined with passing an address of the accessed
variable in the same function invocation. There's no need for the
increments here - simply adding 1 achieves the intended effect without
triggering compiler diagnostics (which are fatal with -Werror).