LeastSquaresSolver: Fix nasty GCC compile optimization error

The original implementation with no wrapping on size_t is more readable
but the compiler errors with:
internal compiler error: in trunc_int_for_mode, at explow.c:55
I read up and it's apparently a loop optimization problem.
Inspired by https://stackoverflow.com/a/27224697/6326048
I used a far less readable implementation that works fine and
wrote a comment to explain it.

matrix/LeastSquaresSolver.hpp

@@ -116,20 +116,21 @@ class LeastSquaresSolver
         Vector<Type, M> qtbv = qtb(b);
         Vector<Type, N> x;
 
-        for (size_t l = N; l > 0 ; l--) {
-            size_t i = l - 1;
+        // size_t is unsigned and wraps i = 0 - 1 to i > N
+        for (size_t i = N - 1; i < N; i--) {
+            printf("i %d\n", static_cast<int>(i));
             x(i) = qtbv(i);
             for (size_t r = i+1; r < N; r++) {
                 x(i) -= _A(i,r) * x(r);
             }
             // divide by zero, return vector of zeros
-            if (fabs(_A(i,i)) < Type(1e-8)) {
+            if (isEqualF(_A(i,i), Type(0), Type(1e-8))) {
                 for (size_t z = 0; z < N; z++) {
                     x(z) = Type(0);
                 }
                 break;
             }
-            x(i) = x(i) / _A(i,i);
+            x(i) /= _A(i,i);
         }
         return x;
     }