Reduce number of variables for arguments in functions

[tomsik68]

Currently, each argument for function gets 2 stack-allocated variables. For example:

int main(int argc, char** argv) {
    int var0;
    char** var1;

    var0 = argc;
    var1 = argv;
}

And the variables of the arguments themselves are not used anymore. We could get rid of variables like var0 and var1.

[tomsik68]

Optimizing the LLVM before decompiling it removes these variables. So it's not too important to solve this directly in llvm2c.