HLSL: Add conversions for image ops during SPV construction

HLSL allows image and texture types to be templatized on sub-vec4 types,
or even structures.  This was mostly handled already during creation of
sampling operations.  However, for operator[] which can generate image
loads, this wasn't happening.

It also isn't very easy to do at that point in time, because operator[]
does not know where the results it produces will end up.  They may be
an lvalue or an rvalue, and there's a post-process to convert loads to
stores.  They may end up in atomic ops.

To bypass that difficulty, GlslangToSpv now looks for this case and
adds the appropriate conversion.  LIMITATION: this only works for
cases for which a simple conversion opcode suffices.  That is to say,
it will not work if the type is templatized on a struct.

Test/hlsl.imagefetch-subvec4.comp

@@ -0,0 +1,8 @@
+Texture3D<int> IN: register(t0);
+RWTexture3D<uint> OUT: register(u1);
+
+[numthreads(8,8,8)]
+void main(uint3 tid: SV_DispatchThreadID)
+{
+    OUT[tid] = IN[tid];
+}