correction of --instrument option support

Summary of these changes:
- we should call function getString with third parameter "false". This function puts our string in string map and it can add null at the end of this. But when we try to read something from string map we search for string without null.
- when we put first parameter in ISPCInstrument function we shouldn't name it's value as filename. Because it will have a conflict in string table with filename. So I added a "_" before this name. It doesn't influence on user interface of this function.
- we should change the type of fourth parameter of function ISPCInstrument in our examples because ispc produces another one.
- in file bitcode2cpp.py i+1 == len(data) but it is better to use len.

These changes were tested on linux.