LiveStacks: Rename LiveStack.{h|cpp} to LiveStacks.{h|cpp}; NFC

Filenames should match the name of the class they contain.

llvm-svn: 321037
diff --git a/llvm/lib/CodeGen/VirtRegMap.cpp b/llvm/lib/CodeGen/VirtRegMap.cpp
index 64bb37a..13f7e83 100644
--- a/llvm/lib/CodeGen/VirtRegMap.cpp
+++ b/llvm/lib/CodeGen/VirtRegMap.cpp
@@ -22,7 +22,7 @@
 #include "llvm/ADT/Statistic.h"
 #include "llvm/CodeGen/LiveInterval.h"
 #include "llvm/CodeGen/LiveIntervals.h"
-#include "llvm/CodeGen/LiveStackAnalysis.h"
+#include "llvm/CodeGen/LiveStacks.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"