Fix issue with separable shader validation in iomapper

Test/iomap.blockOutVariableIn.frag

@@ -0,0 +1,11 @@
+#version 440
+
+layout(location = 0) in vec4 a1;
+layout(location = 1) in vec2 a2;
+
+layout(location = 0) out vec4 color;
+
+void main()
+{
+    color = vec4(a1.xy, a2);
+}