trans_arg_expr: Omit extra copies for rvalues

rvalues aren't going to be used anywhere but as the argument, so
there's no point in copying them. LLVM used to eliminate the copy
later, but why bother emitting it in the first place?
auto merge of #8262 : dotdash/rust/no_rval_copies, r=pcwalton

rvalues aren't going to be used anywhere but as the argument, so
there's no point in copying them. LLVM used to eliminate the copy
later, but why bother emitting it in the first place?