Explicitly USE by-ref arguments of libcalls

[Alexandre Oliva]

When an argument is passed by reference to a libcall in the last basic
block of a function, it is possible that the store be discarded.
libcalls don't introduce basic block boundaries and there's no
explicit use of the values stored in the stack frame, so the store
ends up considered dead by insn_dead_p().  This patch adds the memory
address in which the argument passed by reference to the EXPR_LIST of
USE expressions of the call insn.  Approved by Richard Henderson.