This patch teaches IndVarSimplify to add nuw and nsw to certain kinds

This patch teaches IndVarSimplify to add nuw and nsw to certain kinds
of operations that provably don't overflow. For example, we can prove
%civ.inc below does not sign-overflow. With this change,
IndVarSimplify changes %civ.inc to an add nsw.

  define i32 @foo(i32* %array, i32* %length_ptr, i32 %init) {
   entry:
    %length = load i32* %length_ptr, !range !0
    %len.sub.1 = sub i32 %length, 1
    %upper = icmp slt i32 %init, %len.sub.1
    br i1 %upper, label %loop, label %exit

   loop:
    %civ = phi i32 [ %init, %entry ], [ %civ.inc, %latch ]
    %civ.inc = add i32 %civ, 1
    %cmp = icmp slt i32 %civ.inc, %length
    br i1 %cmp, label %latch, label %break

   latch:
    store i32 0, i32* %array
    %check = icmp slt i32 %civ.inc, %len.sub.1
    br i1 %check, label %loop, label %break

   break:
    ret i32 %civ.inc

   exit:
    ret i32 42
  }

Differential Revision: http://reviews.llvm.org/D6748

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225282 91177308-0d34-0410-b5e6-96231b3b80d8