tree-optimization/113896 - testcase for fixed PR

The SLP permute optimization rewrite fixed this.

	PR tree-optimization/113896
	* g++.dg/torture/pr113896.C: New testcase.

gcc/testsuite/g++.dg/torture/pr113896.C

+// { dg-do run }
+// { dg-additional-options "-ffast-math" }
+
+double a1 = 1.0;
+double a2 = 1.0;
+
+void __attribute__((noipa))
+f(double K[2], bool b)
+{
+    double A[] = {
+        b ? a1 : a2,
+        0,
+        0,
+        0
+    };
+
+    double sum{};
+    for(double  a : A) sum += a;
+    for(double& a : A) a /= sum;
+
+    if (b) {
+        K[0] = A[0]; // 1.0
+        K[1] = A[1]; // 0.0
+    } else {
+        K[0] = A[0] + A[1];
+    }
+}
+
+int main()
+{
+  double K[2]{};
+  f(K, true);
+  if (K[0] != 1. || K[1] != 0.)
+    __builtin_abort ();
+}