[PATCH] Teach IndVarSimplify to add nuw and nsw to operations that provably don't overflow.

[Sanjoy Das]

Hi atrick, majnemer, hfinkel,

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
}

```

http://reviews.llvm.org/D6748

Files:
  lib/Transforms/Utils/SimplifyIndVar.cpp
  test/Transforms/BBVectorize/loop1.ll
  test/Transforms/IndVarSimplify/2011-09-10-widen-nsw.ll
  test/Transforms/IndVarSimplify/strengthen-overflow.ll

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