[SCEV] Factor out utility for proving same sign of two SCEVs [nfc]

This is a slightly different API than ConstantRange's areInsensitiveToSignednessOfICmpPredicate.  The only actual difference (beyond naming) is the handling of empty ranges (i.e. unreachable code).  I wanted to keep the existing SCEV behavior for the unreachable code as we should be folding that to poison, not reasoning about samesign.

The new API will be reused in llvm#170363.  I'll rebase whichever lands second over the first to land.

Author: preames

llvm/include/llvm/Analysis/ScalarEvolution.h

@@ -1078,6 +1078,9 @@ class ScalarEvolution {
   isKnownMultipleOf(const SCEV *S, uint64_t M,
                     SmallVectorImpl<const SCEVPredicate *> &Assumptions);
 
+  /// Return true if we know that S1 and S2 must have the same sign.
+  LLVM_ABI bool haveSameSign(const SCEV *S1, const SCEV *S2);
+
   /// Splits SCEV expression \p S into two SCEVs. One of them is obtained from
   /// \p S by substitution of all AddRec sub-expression related to loop \p L
   /// with initial value of that SCEV. The second is obtained from \p S by

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -11115,6 +11115,11 @@ bool ScalarEvolution::isKnownMultipleOf(
   return true;
 }
 
+bool ScalarEvolution::haveSameSign(const SCEV *S1, const SCEV *S2) {
+  return ((isKnownNonNegative(S1) && isKnownNonNegative(S2)) ||
+          (isKnownNegative(S1) && isKnownNegative(S2)));
+}
+
 std::pair<const SCEV *, const SCEV *>
 ScalarEvolution::SplitIntoInitAndPostInc(const Loop *L, const SCEV *S) {
   // Compute SCEV on entry of loop L.
@@ -12034,8 +12039,7 @@ bool ScalarEvolution::isImpliedCondBalancedTypes(
   if (IsSignFlippedPredicate(Pred, FoundPred)) {
     // Unsigned comparison is the same as signed comparison when both the
     // operands are non-negative or negative.
-    if ((isKnownNonNegative(FoundLHS) && isKnownNonNegative(FoundRHS)) ||
-        (isKnownNegative(FoundLHS) && isKnownNegative(FoundRHS)))
+    if (haveSameSign(FoundLHS, FoundRHS))
       return isImpliedCondOperands(Pred, LHS, RHS, FoundLHS, FoundRHS, CtxI);
     // Create local copies that we can freely swap and canonicalize our
     // conditions to "le/lt".