Implement tagged var liveness analysis on RTL

[yiannist]

Implement a liveness analysis on RTL variables (only tagged) in order to reduce LLVM stack usage. Each RTL tagged var is mapped explicitly to a stack slot (with 'alloca') because it should be tracked by the Garbage Collector (marked with 'gcroot'). A trivial liveness analysis algorithm might significantly improve the performance of ErLLVM (especially in the cases of small recursive functions).