[InstSimplify] Avoid ConstantExpr::getIntegerCast() (NFCI)

This always works on a constant integer or integer splat, so the
constant fold here should always succeed.

Author: nikic

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -895,7 +895,8 @@ static Value *simplifySubInst(Value *Op0, Value *Op1, bool IsNSW, bool IsNUW,
   // Variations on GEP(base, I, ...) - GEP(base, i, ...) -> GEP(null, I-i, ...).
   if (match(Op0, m_PtrToInt(m_Value(X))) && match(Op1, m_PtrToInt(m_Value(Y))))
     if (Constant *Result = computePointerDifference(Q.DL, X, Y))
-      return ConstantExpr::getIntegerCast(Result, Op0->getType(), true);
+      return ConstantFoldIntegerCast(Result, Op0->getType(), /*IsSigned*/ true,
+                                     Q.DL);
 
   // i1 sub -> xor.
   if (MaxRecurse && Op0->getType()->isIntOrIntVectorTy(1))