Teach SCEV's icmp simplification logic that a-b == 0 is equivalent to a == b.

This also required making recursive simplifications until
nothing changes or a hard limit (currently 3) is hit.

With the simplification in place indvars can canonicalize
loops of the form
for (unsigned i = 0; i < a-b; ++i)
into
for (unsigned i = 0; i != a-b; ++i)
which used to fail because SCEV created a weird umax expr
for the backedge taken count.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@157701 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Analysis/ScalarEvolution.h

@@ -837,7 +837,8 @@ namespace llvm {
     ///
     bool SimplifyICmpOperands(ICmpInst::Predicate &Pred,
                               const SCEV *&LHS,
-                              const SCEV *&RHS);
+                              const SCEV *&RHS,
+                              unsigned Depth = 0);
 
     /// getLoopDisposition - Return the "disposition" of the given SCEV with
     /// respect to the given loop.