aarch64, testsuite: Add test for already-fixed PR [PR124491]

This adds a testcase for PR124491, which was fixed by these commits:

r16-6709-ga4716ece529dfd29d169ccc96979f7c747231f25
r16-6743-ge787d5ace5cc315892868f3e5e2a2a16fd31bcc8

I originally just added the test to the 15 branch with the backports for
those commits, but richi pointed out in the PR that we should have it on
trunk too, so this adds it to the trunk.

gcc/testsuite/ChangeLog:

	PR middle-end/124491
	* gcc.target/aarch64/torture/pr124491.c: New test.

gcc/testsuite/gcc.target/aarch64/torture/pr124491.c

@@ -0,0 +1,6 @@
+/* { dg-do compile } */
+/* { dg-options "-march=armv8.2-a+sve -msve-vector-bits=256" } */
+#include <arm_sve.h>
+svint64_t foo(long x, long y) {
+  return svdupq_n_s64(x, y);
+}