It appears that LLVM can sometimes recognize that multiple calls to
runtime.interfaceMethod can be merged into one. When that happens, the
interface lowering pass shows an error as it didn't expect that
situation.
Luckily the fix is very easy.
Panics are bad for usability: whenever something breaks, the user is
shown a (not very informative) backtrace. Replace it with real error
messages instead, that even try to display the Go source location.
