Add regression test for closure loop vectorization

tests/assembly-llvm/slp-vectorize-closure.rs

@@ -0,0 +1,28 @@
+//! Loop inside closure correctly compiled to assembly using SSE instructions.
+//! Regression test for <https://github.com/rust-lang/rust/issues/120189>.
+//! also see <https://godbolt.org/z/W1Yc4s3xo>
+//@ assembly-output: emit-asm
+//@ compile-flags: -Copt-level=3
+//@ only-x86_64
+// to test SSE instructions
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: for_in_closure
+// CHECK: {{paddb|psubb}}
+#[inline(never)]
+#[no_mangle]
+pub fn for_in_closure() {
+    let mut v = [[0u8; 4]; 60];
+
+    let mut closure = || {
+        for item in &mut v {
+            item[0] += 1;
+            item[1] += 1;
+            item[2] += 1;
+            item[3] += 1;
+        }
+    };
+
+    closure();
+}