Add testcase for proper LLVM representation of SIMD types.

Testcase to make sure that no 0-sized padding is inserted in structs and that structs are represented as expected by Neon intrinsics in LLVM.

Author: hkratz

src/test/codegen/unpadded-simd.rs

@@ -0,0 +1,14 @@
+// Make sure that no 0-sized padding is inserted in structs and that
+// structs are represented as expected by Neon intrinsics in LLVM.
+// See #87254.
+
+#![crate_type = "lib"]
+#![feature(repr_simd)]
+
+#[derive(Copy, Clone, Debug)]
+#[repr(simd)]
+pub struct int16x4_t(pub i16, pub i16, pub i16, pub i16);
+
+#[derive(Copy, Clone, Debug)]
+pub struct int16x4x2_t(pub int16x4_t, pub int16x4_t);
+// CHECK: %int16x4x2_t = type { <4 x i16>, <4 x i16> }