target-arm: Simplify insn_crosses_page()

Recent changes have left insn_crosses_page() more complicated
than it needed to be:
 * it's only called from thumb_tr_translate_insn() so we know
   for certain that we're looking at a Thumb insn
 * the caller's check for dc->pc >= dc->next_page_start - 3
   means that dc->pc can't possibly be 4 aligned, so there's
   no need to check that (the check was partly there to ensure
   that we didn't treat an ARM insn as Thumb, I think)
 * we now have thumb_insn_is_16bit() which lets us do a precise
   check of the length of the next insn, rather than opencoding
   an inaccurate check

Simplify it down to just loading the first half of the insn
and calling thumb_insn_is_16bit() on it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1507556919-24992-8-git-send-email-peter.maydell@linaro.org