add codegen test for rust-lang#109328

Author: tesuji

tests/codegen/issues/issue-109328-split_first.rs

@@ -0,0 +1,17 @@
+//@ compile-flags: -O
+//@ min-llvm-version: 17
+
+#![crate_type = "lib"]
+
+#[no_mangle]
+// CHECK-LABEL: @foo
+// CHECK: getelementptr inbounds
+// CHECK-NEXT: load i64
+// CHECK-NEXT: icmp eq i64
+// CHECK-NEXT: br i1
+#[no_mangle]
+pub fn foo(input: &mut &[u64]) -> Option<u64> {
+    let (first, rest) = input.split_first()?;
+    *input = rest;
+    Some(*first)
+}