[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/StraightLineStrengthReduce.cpp b/llvm/lib/Transforms/Scalar/StraightLineStrengthReduce.cpp
index 913454b..1faa65e 100644
--- a/llvm/lib/Transforms/Scalar/StraightLineStrengthReduce.cpp
+++ b/llvm/lib/Transforms/Scalar/StraightLineStrengthReduce.cpp
@@ -524,7 +524,7 @@
       continue;
 
     const SCEV *OrigIndexExpr = IndexExprs[I - 1];
-    IndexExprs[I - 1] = SE->getConstant(OrigIndexExpr->getType(), 0);
+    IndexExprs[I - 1] = SE->getZero(OrigIndexExpr->getType());
 
     // The base of this candidate is GEP's base plus the offsets of all
     // indices except this current one.