vect: add testcase from [PR123032]

Adding tests to testsuite, fixed by gcc-16-6031-g8aa4ef38bd1

gcc/testsuite/ChangeLog:

	PR tree-optimization/123032
	* gcc.dg/vect/pr123032.c: New test.

gcc/testsuite/gcc.dg/vect/pr123032.c

@@ -0,0 +1,55 @@
+/* { dg-require-effective-target vect_int } */
+/* { dg-final { scan-tree-dump "loop vectorized" "vect" } } */
+
+#include "tree-vect.h"
+
+typedef unsigned char u8;
+typedef unsigned int u32;
+
+typedef struct {
+    u32 pad; // important
+    u8  buf[64];
+} s_t;
+
+__attribute__((noipa))
+void h_bad(s_t *state, const u8 *in, u32 inlen) {
+    for (u32 i = 0; i < inlen; i++) {
+        state->buf[i] = in[i];
+    }
+}
+
+__attribute__((noipa))
+void h_good(s_t *state, const u8 *in, u32 inlen) {
+    for (u32 i = 0; i < inlen; i++) {
+        asm volatile("" ::: "memory"); // break vectorizer
+        state->buf[i] = in[i];
+    }
+}
+
+void h(s_t *state, const u8 *in, u32 inlen) {
+    s_t s1 = {};
+    s_t s2 = {};
+    h_good(&s1, in, inlen);
+    h_bad(&s2, in, inlen);
+    if (__builtin_memcmp(&s1, &s2, sizeof(s1)) != 0) {
+        __builtin_trap();
+    }
+}
+
+int main(void) {
+
+   check_vect ();
+
+    s_t s = {};
+    const u8 in[] =
+        "0123456789" /* 0 */
+        " 0123456789"
+        " 0123456789"
+        " 0123456789"
+        " 0123456789"
+    ;
+    for (u32 o = 0; o < 10; o++) {
+        __builtin_memset(&s, 0, sizeof(s));
+        h(&s, &in[o], 33);
+    }
+}