Finish the new converter of LLVM IR into BIR

[s3rvac]

Currently, there are two converters of LLVM IR into BIR in llvmir2hll (see source code in src/llvmir2hll/llvm/llvmir2bir_converters):

• orig Original converter, which has been in the decompiler since its beginning.
• new A complete rewrite of the converter.

The used converter can be set via --backend-llvmir2bir-converter when running retdec-decompiler.sh. Currently, the default one is orig, because the new converter is not finished.

We should finish the converter. It has the following two main issues:

• It generates goto targets without labels (e.g. goto xyz; is generated, but there is no xyz label in the code).
• It generates more goto statements than the original converter.

A word of caution: Several people have already tried to finish the new converter, but none of them succeeded. It is not an easy task.

[PeterMatula]

The default LLVM IR to BIR converter was changed in e6f47cf. This commit is already in master.