There are only two remaining uses of gen_bx_im.  In each case, we
know the destination mode -- not changing in the case of gen_jmp
or changing in the case of trans_BLX_i.  Use this to simplify the
surrounding code.

For trans_BLX_i, use gen_jmp for the actual branch.  For gen_jmp,
use gen_set_pc_im to set up the single-step.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190904193059.26202-70-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>