Avoid producing invalid iterators in division loops

This decouples loop end expression from the iterator decrement.
Previously, the `it--` produced invalid iterator to `u[-1]`
in the last iteration. It might have not been a big deal for execution
because the loop terminated and the iterator was unused,
but this was theoretical undefined behavior and constexpr context
didn't like it either.

include/intx/intx.hpp

@@ -1605,10 +1605,13 @@ inline uint64_t udivrem_by1(uint64_t u[], int len, uint64_t d) noexcept
     u[len - 1] = 0;         // Reset the word being a part of the result quotient.
 
     auto it = &u[len - 2];
-    do
+    while (true)
     {
         std::tie(*it, rem) = udivrem_2by1({*it, rem}, d, reciprocal);
-    } while (it-- != &u[0]);
+        if (it == &u[0])
+            break;
+        --it;
+    }
 
     return rem;
 }
@@ -1629,10 +1632,13 @@ inline uint128 udivrem_by2(uint64_t u[], int len, uint128 d) noexcept
     u[len - 1] = u[len - 2] = 0;  // Reset these words being a part of the result quotient.
 
     auto it = &u[len - 3];
-    do
+    while (true)
     {
         std::tie(*it, rem) = udivrem_3by2(rem[1], rem[0], *it, d, reciprocal);
-    } while (it-- != &u[0]);
+        if (it == &u[0])
+            break;
+        --it;
+    }
 
     return rem;
 }