Fix cast in AffineMap::getSingleConstantValue and rename to
getSingleConstantResult.

PiperOrigin-RevId: 212544394
diff --git a/lib/IR/Statement.cpp b/lib/IR/Statement.cpp
index 1f7604f..c8d3336 100644
--- a/lib/IR/Statement.cpp
+++ b/lib/IR/Statement.cpp
@@ -336,11 +336,11 @@
 }
 
 int64_t ForStmt::getConstantLowerBound() const {
-  return lbMap->getSingleConstantValue();
+  return lbMap->getSingleConstantResult();
 }
 
 int64_t ForStmt::getConstantUpperBound() const {
-  return ubMap->getSingleConstantValue();
+  return ubMap->getSingleConstantResult();
 }
 
 Optional<uint64_t> ForStmt::getConstantTripCount() const {