Decorate accesschain operand for nonuniform UBO loads

This is conservative and still also decorates the loaded value.

Test/spv.nonuniform5.frag

@@ -0,0 +1,15 @@
+#version 450
+#extension GL_EXT_nonuniform_qualifier : require
+
+layout(location = 0) flat in int Index;
+layout(location = 0) out vec4 FragColor;
+
+layout(set = 0, binding = 0) uniform UBO
+{
+	vec4 v;
+} ubos[];
+
+void main()
+{
+	FragColor = ubos[nonuniformEXT(Index)].v;
+}