Add support to indvars for optimizing sadd.with.overflow.

Split sadd.with.overflow into add + sadd.with.overflow to allow
analysis and optimization. This should ideally be done after
InstCombine, which can perform code motion (eventually indvars should
run after all canonical instcombines). We want ISEL to recombine the
add and the check, at least on x86.

This is currently under an option for reducing live induction
variables: -liv-reduce. The next step is reducing liveness of IVs that
are live out of the overflow check paths. Once the related
optimizations are fully developed, reviewed and tested, I do expect
this to become default.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197926 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Transforms/Utils/SimplifyIndVar.h

@@ -22,6 +22,7 @@
 namespace llvm {
 
 class CastInst;
+class DominatorTree;
 class IVUsers;
 class Loop;
 class LPPassManager;
@@ -31,9 +32,25 @@ class ScalarEvolution;
 /// Interface for visiting interesting IV users that are recognized but not
 /// simplified by this utility.
 class IVVisitor {
+protected:
+  const DominatorTree *DT;
+  bool ShouldSplitOverflowIntrinsics;
+
   virtual void anchor();
 public:
+  IVVisitor(): DT(NULL), ShouldSplitOverflowIntrinsics(false) {}
   virtual ~IVVisitor() {}
+
+  const DominatorTree *getDomTree() const { return DT; }
+
+  bool shouldSplitOverflowInstrinsics() const {
+    return ShouldSplitOverflowIntrinsics;
+  }
+  void setSplitOverflowIntrinsics() {
+    ShouldSplitOverflowIntrinsics = true;
+    assert(DT && "Splitting overflow intrinsics requires a DomTree.");
+  }
+
   virtual void visitCast(CastInst *Cast) = 0;
 };