Front-ends GLSL/HLSL: Fix initializer lists for structs of one member.

Single member structs initialized with an initializer list had
an incorrect argument for constructor emulation.

Test/hlsl.init.frag

@@ -4,6 +4,18 @@ float a2 = 0.2, b2;
 float a3, b3 = 0.3;
 float a4, b4 = 0.4, c4;
 float a5 = 0.5, b5, c5 = 1.5;
+
+struct Single1 { int f; };
+Single1 single1 = { 10 };
+
+struct Single2 { uint2 v; };
+Single2 single2 = { { 1, 2 } };
+
+struct Single3 { Single1 s1; };
+Single3 single3 = { { 3 } };
+
+struct Single4 { Single2 s1; };
+Single4 single4 = { { { 4u, 5u } } };
 
 float4 ShaderFunction(float4 input) : COLOR0
 {