LLVM definitely uses IEEE-754 for its floating point types, other than the PPC double double format.
Although the LLVM langref now makes the data types IEEE-754 binary formats the floating-point instructions themselves are still very under specified. For example, fadd doesn't mention what the semantics of the additional are (e.g. IEEE-754 addition). I suspect the reasons for that are the PPC double double format and that it would be very unusual to use a IEEE-754 binary encoding but then not perform IEEE-754 operations on that data. However I think LLVM really should be more explicit.
This is probably a bit off topic though... one for llvm-dev maybe?