testsuite: torture: add LLVM testcase for DSE vs. -ftrivial-auto-var-init=

This testcase came up in a recent LLVM bug report [0] for DSE vs
-ftrivial-auto-var-init=. Add it to our testsuite given that area
could do with better coverage.

[0] https://github.com/llvm/llvm-project/issues/119646



gcc/testsuite/ChangeLog:

	* gcc.dg/torture/dse-trivial-auto-var-init.c: New test.

Co-authored-by: Andrew Pinski <pinskia@gmail.com>

gcc/testsuite/gcc.dg/torture/dse-trivial-auto-var-init.c

+/* Testcase for LLVM bug: https://github.com/llvm/llvm-project/issues/119646 */
+/* { dg-do run } */
+/* { dg-additional-options "-ftrivial-auto-var-init=zero" } */
+
+int b = 208;
+[[gnu::noinline]]
+void f(int *e, int a) {
+  *e = !!b;
+  if (a)
+    __builtin_trap();
+}
+int main(void) {
+  b = 0;
+  f(&b, 0);
+  if (b != 0)
+    __builtin_trap();
+}