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.

Backports commit 5b8d7289e9e92a0d7bcecb93cd189e245fef10cd from qemu

qemu/target/arm/translate.c

@@ -12083,29 +12083,14 @@ static bool insn_crosses_page(CPUARMState *env, DisasContext *s)
 {
     /* Return true if the insn at dc->pc might cross a page boundary.
      * (False positives are OK, false negatives are not.)
+     * We know this is a Thumb insn, and our caller ensures we are
+     * only called if dc->pc is less than 4 bytes from the page
+     * boundary, so we cross the page if the first 16 bits indicate
+     * that this is a 32 bit insn.
      */
-    uint16_t insn;
+    uint16_t insn = arm_lduw_code(env, s->pc, s->sctlr_b);
 
-    if ((s->pc & 3) == 0) {
+    return !thumb_insn_is_16bit(s, insn);
-        /* At a 4-aligned address we can't be crossing a page */
-        return false;
-    }
-
-    /* This must be a Thumb insn */
-    insn = arm_lduw_code(env, s->pc, s->sctlr_b);
-
-    if ((insn >> 11) >= 0x1d) {
-        /* Top five bits 0b11101 / 0b11110 / 0b11111 : this is the
-         * First half of a 32-bit Thumb insn. Thumb-1 cores might
-         * end up actually treating this as two 16-bit insns (see the
-         * code at the start of disas_thumb2_insn()) but we don't bother
-         * to check for that as it is unlikely, and false positives here
-         * are harmless.
-         */
-        return true;
-    }
-    /* Definitely a 16-bit insn, can't be crossing a page. */
-    return false;
 }
 
 static int arm_tr_init_disas_context(DisasContextBase *dcbase,