Add codegen test for SLP vectorization

tests/codegen-llvm/slp-vectorization-mul3.rs

@@ -0,0 +1,26 @@
+//! Regression test for #142519
+//@ only-x86_64
+//@ compile-flags: -O
+//@ min-llvm-version: 22
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: @mul3
+// CHECK: phi <4 x i8>
+// CHECK: load <4 x i8>
+// CHECK: add <4 x i8>
+// CHECK: store <4 x i8>
+
+#[no_mangle]
+pub fn mul3(previous: &[[u8; 4]], current: &mut [[u8; 4]]) {
+    let mut c_bpp = [0u8; 4];
+
+    for i in 0..previous.len() {
+        current[i][0] = current[i][0].wrapping_add(c_bpp[0]);
+        current[i][1] = current[i][1].wrapping_add(c_bpp[1]);
+        current[i][2] = current[i][2].wrapping_add(c_bpp[2]);
+        current[i][3] = current[i][3].wrapping_add(c_bpp[3]);
+
+        c_bpp = previous[i];
+    }
+}