Allow nonuniformEXT() on sampler types.

Also add missing NonUniformEXT decoration on OpImageTexelPointer.

Test/spv.nonuniform3.frag

@@ -0,0 +1,10 @@
+#version 450
+#extension GL_EXT_nonuniform_qualifier : require
+layout(set = 0, binding = 0) uniform texture2D uTex[];
+layout(set = 1, binding = 0) uniform sampler uSamp;
+layout(location = 0) flat in int Index;
+layout(location = 0) out vec4 FragColor;
+void main()
+{
+    FragColor = texture(nonuniformEXT(sampler2D(uTex[Index], uSamp)), vec2(0.5));
+}