We were (unintentionally) only using structural equivalence to compare struct types.

Now we require that the struct name match for two struct types to be the same.
Added a test to check this.
(Also removed a stale test, movmsk-opt.ispc)

tests_errors/struct_type_equality.ispc

@@ -0,0 +1,14 @@
+// Can't convert between different struct types "uniform struct Foo" and "uniform struct Bar"
+
+struct Foo {
+    int a; 
+};
+
+struct Bar {
+    int x; 
+};
+
+void f(Foo a) { 
+    Bar b;
+    b  =a;
+}