target/i386: pull load/writeback out of gen_shiftd_rm_T1

Use gen_ld_modrm/gen_st_modrm, moving them and gen_shift_flags to the
caller.  This way, gen_shiftd_rm_T1 becomes something that the new
decoder can call.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>