tweak the comment

compiler/rustc_codegen_ssa/src/mir/operand.rs

@@ -637,9 +637,11 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
 
             mir::Operand::Constant(ref constant) => {
                 let constant_ty = self.monomorphize(constant.ty());
-                // Make sure this type is actually passed as an immediate vector.
-                // (In particular, this excludes packed SIMD vectors.)
+                // Most SIMD vector constants should be passed as immediates.
+                // (In particular, some intrinsics really rely on this.)
                 if constant_ty.is_simd() {
+                    // However, some SIMD types do not actually use the vector ABI
+                    // (in particular, packed SIMD types do not). Ensure we exclude those.
                     let layout = bx.layout_of(constant_ty);
                     if let Abi::Vector { .. } = layout.abi {
                         let (llval, ty) = self.immediate_const_vector(bx, constant);