Add test for new delayed bug code path

tests/ui/simd/monomorphize-heterogeneous.rs

@@ -0,0 +1,9 @@
+#![feature(repr_simd)]
+
+#[repr(simd)]
+struct I64F64(i64, f64);
+//~^ ERROR SIMD vector should be homogeneous
+
+static X: I64F64 = I64F64(1, 2.0);
+
+fn main() {}

tests/ui/simd/monomorphize-heterogeneous.stderr

@@ -0,0 +1,9 @@
+error[E0076]: SIMD vector should be homogeneous
+  --> $DIR/monomorphize-heterogeneous.rs:4:1
+   |
+LL | struct I64F64(i64, f64);
+   | ^^^^^^^^^^^^^ SIMD elements must have the same type
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0076`.