[InstCombine] Canonicalize icmp instructions based on dominating conditions.

Summary:
    This patch canonicalizes conditions based on the constant range information
    of the dominating branch condition.
    For example:

      %cmp = icmp slt i64 %a, 0
      br i1 %cmp, label %land.lhs.true, label %lor.rhs
      lor.rhs:
        %cmp2 = icmp sgt i64 %a, 0

    Would now be canonicalized into:

      %cmp = icmp slt i64 %a, 0
      br i1 %cmp, label %land.lhs.true, label %lor.rhs
      lor.rhs:
        %cmp2 = icmp ne i64 %a, 0

Reviewers: mcrosier, gberry, t.p.northover, llvm-commits, reames, hfinkel, sanjoy, majnemer

Subscribers: MatzeB, majnemer, mcrosier

Differential Revision: http://reviews.llvm.org/D18841

llvm-svn: 268521