compiler: avoid introducing redundant write barriers

compiler: avoid introducing redundant write barriers

    The traversal used by the write barrier insertion phase can sometimes
    wind up visiting new statements inserted during the traversal, which
    then results in duplicate / redundant write barrier guards. Example
    program to reproduce:

      package small
      type S struct {
            N *S
            K int
      }
      var G *S = &S{N: nil, K: 101}

    This patch changes the traversal code to keep track of statements
    already added and avoid processing them again later in the traversal.

    Fixes golang/go#25867

    Reviewed-on: https://go-review.googlesource.com/118637

From-SVN: r261568