[Attributor][FIX] Do not treat byval args as local memory (for now)

For now we do should not treat byval arguments as local copies performed
on the call edge, though, in general we should. To make that happen we
need to teach various passes, e.g., DSE, about the copy effect of a
byval. That would also allow us to mark functions only accessing byval
arguments as readnone again, atguably their acceses have no effect
outside of the function, like accesses to allocas.

Reviewed By: kuter

Differential Revision: https://reviews.llvm.org/D108140