Allow nonuniformEXT() on sampler types.

Also add missing NonUniformEXT decoration on OpImageTexelPointer.

Test/spv.nonuniform4.frag

@@ -0,0 +1,8 @@
+#version 450
+#extension GL_EXT_nonuniform_qualifier : require
+layout(set=0,binding=4,r32ui) uniform uimageBuffer data[];
+layout(location = 3) in flat int  rIndex;
+void main()
+{
+  imageAtomicAdd(data[nonuniformEXT(rIndex)], 0, 0);
+}