[IndVars] Widen more comparisons for non-negative induction vars

[IndVars] Widen more comparisons for non-negative induction vars

Summary:
If an induction variable is provably non-negative, its sign extension is
equal to its zero extension.  This means narrow uses like

  icmp slt iNarrow %indvar, %rhs

can be widened into

  icmp slt iWide zext(%indvar), sext(%rhs)

Reviewers: atrick, mcrosier, hfinkel

Subscribers: hfinkel, reames, llvm-commits

Differential Revision: http://reviews.llvm.org/D12745

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248045 91177308-0d34-0410-b5e6-96231b3b80d8