Allow parsing of updated load_ti instructions.

[ptersilie]

#1318 changes the representation of the load_ti instruction from load_ti <offset> to load_ti <VarLocationIdx>. Printing such an instruction results in output like load_ti VarLocation::Register(GP(RAX)). This change, however, hasn't been applied to the JIT IR parser yet. I've worked around this by cheating a bit: the parser still parses load_ti <usize> and then just makes up a VarLocation (always GP(RBX))).

Unfortunately, this means that the input and ouput of the JIT IR parser no longer match, and thus we had to change some tests and disable the roundtrip test. A proper solution for this would be to update the JIT IR parser to be able to parse load_ti instructions such as load_ti Register(GP(RAX)) or load_ti Direct(off, size) or load_ti Indirect(off, size), etc.