MachineSSAUpdater: Allow initialization with just a register class

The register class is required for inserting PHIs, but the "current
virtual register" isn't actually used for anything, so let's remove it
while we're at it.

Differential Revision: https://reviews.llvm.org/D85602

Change-Id: I1e647f31570ef21a7ea8e20db3454178e98a6a8b

Author: nhaehnle

llvm/include/llvm/CodeGen/MachineSSAUpdater.h

@@ -40,9 +40,6 @@ class MachineSSAUpdater {
   //typedef DenseMap<MachineBasicBlock*, Register> AvailableValsTy;
   void *AV = nullptr;
 
-  /// VR - Current virtual register whose uses are being updated.
-  Register VR;
-
   /// VRC - Register class of the current virtual register.
   const TargetRegisterClass *VRC;
 
@@ -65,6 +62,7 @@ class MachineSSAUpdater {
   /// Initialize - Reset this object to get ready for a new set of SSA
   /// updates.
   void Initialize(Register V);
+  void Initialize(const TargetRegisterClass *RC);
 
   /// AddAvailableValue - Indicate that a rewritten value is available at the
   /// end of the specified block with the specified value.

llvm/lib/CodeGen/MachineSSAUpdater.cpp

@@ -50,15 +50,18 @@ MachineSSAUpdater::~MachineSSAUpdater() {
 }
 
 /// Initialize - Reset this object to get ready for a new set of SSA
-/// updates.  ProtoValue is the value used to name PHI nodes.
-void MachineSSAUpdater::Initialize(Register V) {
+/// updates.
+void MachineSSAUpdater::Initialize(const TargetRegisterClass *RC) {
   if (!AV)
     AV = new AvailableValsTy();
   else
     getAvailableVals(AV).clear();
 
-  VR = V;
-  VRC = MRI->getRegClass(VR);
+  VRC = RC;
+}
+
+void MachineSSAUpdater::Initialize(Register V) {
+  Initialize(MRI->getRegClass(V));
 }
 
 /// HasValueForBlock - Return true if the MachineSSAUpdater already has a value for