translate-all: iterate over TBs in a page with PAGE_FOR_EACH_TB

This commit does several things, but to avoid churn I merged them all
into the same commit. To wit:

- Use uintptr_t instead of TranslationBlock * for the list of TBs in a page.
  Just like we did in (c37e6d7e "tcg: Use uintptr_t type for
  jmp_list_{next|first} fields of TB"), the rationale is the same: these
  are tagged pointers, not pointers. So use a more appropriate type.

- Only check the least significant bit of the tagged pointers. Masking
  with 3/~3 is unnecessary and confusing.

- Introduce the TB_FOR_EACH_TAGGED macro, and use it to define
  PAGE_FOR_EACH_TB, which improves readability. Note that
  TB_FOR_EACH_TAGGED will gain another user in a subsequent patch.

- Update tb_page_remove to use PAGE_FOR_EACH_TB. In case there
  is a bug and we attempt to remove a TB that is not in the list, instead
  of segfaulting (since the list is NULL-terminated) we will reach
  g_assert_not_reached().

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

include/exec/exec-all.h

@@ -360,7 +360,7 @@ struct TranslationBlock {
     struct TranslationBlock *orig_tb;
     /* first and second physical page containing code. The lower bit
        of the pointer tells the index in page_next[] */
-    struct TranslationBlock *page_next[2];
+    uintptr_t page_next[2];
     tb_page_addr_t page_addr[2];
 
     /* The following data are used to directly call another TB from