test(std): Add codegen test for array::from_fn optimization

This commit adds a new test file 'array-from_fn.rs' to the codegen test suite.
The test checks the behavior of std::array::from_fn under different optimization levels:

1. At opt-level=0 (debug build), it verifies that the core::array::Guard
   is present in the generated code.
2. At opt-level=s (size optimization), it ensures that the Guard is
   optimized out.

This test helps ensure that the compiler correctly optimizes array::from_fn
calls in release builds while maintaining safety checks in debug builds.

tests/codegen/array-from_fn.rs

@@ -0,0 +1,13 @@
+//@ revisions: NORMAL OPT
+//@ [NORMAL] compile-flags: -C opt-level=0 -C debuginfo=2
+//@ [OPT] compile-flags: -C opt-level=s -C debuginfo=0
+
+#![crate_type = "lib"]
+#![feature(array_from_fn)]
+
+#[no_mangle]
+pub fn iota() -> [u8; 16] {
+    // OPT-NOT: core..array..Guard
+    // NORMAL: core..array..Guard
+    std::array::from_fn(|i| i as _)
+}