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()