Currently unnamed function arguments get assigned the first available numbers %0, %1, ... and developers are just expected to know that and count arguments if needed. It would be better to print out the numbers explicitly, like we do for instructions in the body.
It's worth noting that updating front-ends' tests will probably be a bit of work, but I've got a (embarrassingly bad) script that gets about 80-90% of the way there. I'll reply to any commit with it attached.
I think you need a space before this string? Although, then shouldn't llvm/unittests/IR/AsmWriterTest.cpp be failing? (it has a space there...)