HLSL: Add EOpMatrixSwizzle, selectively decomposed to other ops, for issue #670.

Since EOpMatrixSwizzle is a new op, existing back-ends only work when the
front end first decomposes it to other operations. So far, this is only
being done for simple assignment into matrix swizzles.

SPIRV/GlslangToSpv.cpp

@@ -1115,6 +1115,9 @@ bool TGlslangToSpvTraverser::visitBinary(glslang::TVisit /* visit */, glslang::T
             builder.accessChainPushSwizzle(swizzle, convertGlslangToSpvType(node->getLeft()->getType()));
         }
         return false;
+    case glslang::EOpMatrixSwizzle:
+        logger->missingFunctionality("matrix swizzle");
+        return true;
     case glslang::EOpLogicalOr:
     case glslang::EOpLogicalAnd:
         {