Add tests of things the docs claim will cause an error to be issued

tests_errors/vec-different-size-assign.ispc

@@ -0,0 +1,5 @@
+// Can't convert between differently sized vector types
+
+void foo(float<3> a, int<2> b) {
+    a += b;
+}

tests_errors/vec-size-compile-constant.ispc

@@ -0,0 +1,5 @@
+// Syntax error--token "i" unexpected
+
+void foo(uniform int i) {
+    float<i> a;
+}