Prior to this patch, libcalls inserted by the `SelectionDAG` legalizer
could never be tailcalled. The eligibility of libcalls for tail calling
is is partly determined by checking
`TargetLowering::isInTailCallPosition` and comparing the return type of
the libcall and the caller. `isInTailCallPosition` in turn calls
`TargetLowering::isUsedByReturnOnly` (which always returns false if not
implemented by the target).
