Fix spec const construtor for matrix and vector

Fix issue: #237

1. The code generated for matrix constructor should 1) build column
vectors first, 2) build matrix with the vectors.

2. When there is only one scalar type constituent in vector's
constructor, we should populate the constituent to fill all the slots in
the vector. As for matrix, the single constituent should be populated to
the diagonal positions (top-left to bottom-right diagonal).

remove createSpvConstantFromConstSubTree()

Test/spv.specConstantComposite.vert

@@ -42,6 +42,11 @@ const vec4 spec_vec4_all_spec =
     vec4(spec_float, spec_float, spec_float, spec_float);
 const vec4 spec_vec4_partial_spec =
     vec4(spec_float, spec_float, 300.14, 300.14);
+const vec4 spec_vec4_from_one_scalar = vec4(spec_float);
+
+// Matrix constructor
+const mat2x3 spec_mat2x3 = mat2x3(spec_float, spec_float, spec_float, 1.1, 2.2, 3.3);
+const mat2x3 spec_mat2x3_from_one_scalar = mat2x3(spec_float);
 
 // Struct nesting constructor
 const nesting_struct spec_nesting_struct_all_spec = {