This is a common case, but it also complicates the code. Removing this
special case does have a negative effect on code size in rare cases, but
I don't think it's worth keeping around (and possibly causing bugs) for
such uncommon cases.
This should not result in functional changes, although the output (as
stated above) sometimes changes a little bit.
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.
