LiveRegUnits: Rename accumulateBackward()->accumulate()

Contrary to the stepForward()/stepBackward() method accumulate() doesn't
have a direction as defs, uses and clobbers all have the same effect.

Also improve the documentation comment.

llvm-svn: 307351
diff --git a/llvm/lib/CodeGen/LiveRegUnits.cpp b/llvm/lib/CodeGen/LiveRegUnits.cpp
index 3746b74..f9ba4ff 100644
--- a/llvm/lib/CodeGen/LiveRegUnits.cpp
+++ b/llvm/lib/CodeGen/LiveRegUnits.cpp
@@ -67,7 +67,7 @@
   }
 }
 
-void LiveRegUnits::accumulateBackward(const MachineInstr &MI) {
+void LiveRegUnits::accumulate(const MachineInstr &MI) {
   // Add defs, uses and regmask clobbers to the set.
   for (ConstMIBundleOperands O(MI); O.isValid(); ++O) {
     if (O->isReg()) {