Separating compilation information from IR data structures

It is poorly designed that the data structures for IR also contains information that is gradually generated during compilation, such as:

• use_count and expr in SSAVarEntry
• block_trace in MuFunctionVersion
• exception_blocks in FunctionContent
• control_flow in Block

They are initially not available, and are generated during compilation. They can be safely destroyed after the compilation. These compilation information should be stored separately from the IR.