Only try swizzles on vectors, numbers, and booleans.

Test/invalidSwizzle.vert

@@ -1,8 +1,10 @@
 #version 420
 
 void f();
+uniform sampler2D s;
 
 void main() {
+    vec2 v = s.rr; // Swizzles do not apply to samplers
     f().xx; // Scalar swizzle does not apply to void
     f().xy; // Vector swizzle does not apply either
 }