Instead of having a vector of unique_ptr stored in a vector and
returning star pointers to this, use shared_ptr. While changing
initialization code, move it to a separate file when possible.

This is a first step to allow code analysis and node generation beyond
the ShaderIR class.

It's not always used as a basic block. Rename it for consistency.

Some games call LDG at the top of a basic block, making the tracking
heuristic to fail. This commit lets the heuristic the decoded nodes as a
whole instead of per basic blocks.

This may lead to some false positives but allows it the heuristic to
track cases it previously couldn't.

std::moveing a local variable in a return statement has the potential to
prevent copy elision from occurring, so this can just be converted into
a regular return.