[patch] optimize std::list move constructor

[Jonathan Wakely]

While preparing the initial commit for the std::list ABI transition I
noticed the move constructor does unnecessary work, initializing the
object to an empty state, then calling _List_node_base::swap which
checks whether it's empty (but we already know it is) and then
initializes it again. The _List_node_base::swap function is a call
into libstdc++.so so can't be inlined or otherwise optimized.

This patch changes the move constructor to avoid unnecessary checking
and redundant initializations. The code size increases for -Os, but on
my laptop the following silly test goes from 8s to 4.5s with -O3, and
from 10.5s to 8.5s with -Os, so I think it's worthwhile:

  #include <list>

  int main()
  {
    std::list<int> l1;
    for (long i = 0; i < 1000000000; ++i)
    {
      std::list<int> l2 = std::move(l1);
      l1 = std::move(l2);
    }
  }

I intend to commit this to trunk later today.

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