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.

gtests/Hlsl.FromFile.cpp

@@ -189,6 +189,7 @@ INSTANTIATE_TEST_CASE_P(
         {"hlsl.hull.ctrlpt-2.tesc", "main"},
         {"hlsl.identifier.sample.frag", "main"},
         {"hlsl.if.frag", "PixelShaderFunction"},
+        {"hlsl.imagefetch-subvec4.comp", "main"},
         {"hlsl.implicitBool.frag", "main"},
         {"hlsl.inf.vert", "main"},
         {"hlsl.inoutquals.frag", "main"},