While changing this code, simplify tracking code to allow returning
the base address node, this way callers don't have to manually rebuild
it on each invocation.
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.
Some games search conditionally use global memory instructions. This
allows the heuristic to search inside conditional nodes for the source
constant buffer.
