tcg/riscv: Remove MO_BSWAP handling

TCG_TARGET_HAS_MEMORY_BSWAP is already unset for this backend,
which means that MO_BSWAP be handled by the middle-end and
will never be seen by the backend.  Thus the indexes used with
qemu_{ld,st}_helpers will always be zero.

Tidy the comments and asserts in tcg_out_qemu_{ld,st}_direct.
It is not that we do not handle bswap "yet", but never will.

Acked-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>