Allow llvm_assert_eq to be used with function arguments

[atomb]

At the moment, if you say llvm_assert_eq "x" {{ ... }}, where x is a function argument, you'll get the error message "argument assignments not allowed". There's no deep reason why it couldn't be supported, though.

[atomb]

This will be best supported by the refactoring happening in the llvm_verify_exp branch. That work won't be done by the 0.2-alpha release, unfortunately, so I'm removing that milestone from this ticket.

[atomb]

As it turns out, we managed to merge the llvm_verify_exp branch into the 0.2 release. On top of those changes, commit cf0e50f fixes the issue with llvm_assert_eq and function arguments (though this unfortunately didn't make the release).