Extend RTL API to support the LLVM backend

Extend the 'rtl_var' definition to host liveness information needed for
a simple liveness analysis performed by the LLVM backend.

Also, uncomment some function definitions (mostly simple accessors) that
are already there and are useful for the translation of RTL code to LLVM
assembly.

Finally, extend the RTL 'call' instruction with the 'normalcontinuation'
field which is required for translating calls that are in the scope of
an exception handler. This extra field is required in order to point to
a new basic block which will hold the 'unwind label' of LLVM's invoke
instruction. While the 'unwind label' is semantically equivalent with
the 'failcontinuation', in LLVM this block must have a 'landingpad'
instruction. The problem arises by the fact that an RTL 'continuation'
block can also be accessible by other paths in the RTL CFG, and so
cannot be marked as a landing pad. To overcome this issue we create a
new block (the 'normalcontinuation') which is used as the 'unwind label'
of LLVM's invoke instruction and which will eventually transfer control
to the 'continuation' block.

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