[SCEV] Introduce ScalarEvolution::getOne and getZero.

Summary:
It is fairly common to call SE->getConstant(Ty, 0) or
SE->getConstant(Ty, 1); this change makes such uses a little bit
briefer.

I've refactored the call sites I could find easily to use getZero /
getOne.

Reviewers: hfinkel, majnemer, reames

Subscribers: sanjoy, llvm-commits

Differential Revision: http://reviews.llvm.org/D12947

llvm-svn: 248362
diff --git a/llvm/lib/Transforms/Scalar/AlignmentFromAssumptions.cpp b/llvm/lib/Transforms/Scalar/AlignmentFromAssumptions.cpp
index 3c2c0cb..a13e552 100644
--- a/llvm/lib/Transforms/Scalar/AlignmentFromAssumptions.cpp
+++ b/llvm/lib/Transforms/Scalar/AlignmentFromAssumptions.cpp
@@ -270,7 +270,7 @@
   OffSCEV = nullptr;
   if (PtrToIntInst *PToI = dyn_cast<PtrToIntInst>(AndLHS)) {
     AAPtr = PToI->getPointerOperand();
-    OffSCEV = SE->getConstant(Int64Ty, 0);
+    OffSCEV = SE->getZero(Int64Ty);
   } else if (const SCEVAddExpr* AndLHSAddSCEV =
              dyn_cast<SCEVAddExpr>(AndLHSSCEV)) {
     // Try to find the ptrtoint; subtract it and the rest is the offset.