add codegen test for #112509

Author: tesuji

tests/codegen/issues/issue-112509-slice-get-andthen-get.rs

@@ -0,0 +1,17 @@
+//@ compile-flags: -O
+//@ min-llvm-version: 17
+#![crate_type = "lib"]
+
+// CHECK-LABEL: @write_u8_variant_a
+// CHECK: getelementptr
+// CHECK-NEXT: icmp ugt
+#[no_mangle]
+pub fn write_u8_variant_a(
+    bytes: &mut [u8],
+    buf: u8,
+    offset: usize,
+) -> Option<&mut [u8]> {
+    let buf = buf.to_le_bytes();
+    bytes
+        .get_mut(offset..).and_then(|bytes| bytes.get_mut(..buf.len()))
+}