MachineFunction: Remove null check on TargetRegisterInfo (#128480)

Targets are required to define this, it is not optional. Make the method
pure virtual to enforce this

Author: arsenm

llvm/include/llvm/CodeGen/TargetSubtargetInfo.h

@@ -124,9 +124,8 @@ class TargetSubtargetInfo : public MCSubtargetInfo {
 
   virtual const LegalizerInfo *getLegalizerInfo() const { return nullptr; }
 
-  /// getRegisterInfo - If register information is available, return it.  If
-  /// not, return null.
-  virtual const TargetRegisterInfo *getRegisterInfo() const { return nullptr; }
+  /// Return the target's register information.
+  virtual const TargetRegisterInfo *getRegisterInfo() const = 0;
 
   /// If the information for the register banks is available, return it.
   /// Otherwise return nullptr.

llvm/lib/CodeGen/MachineFunction.cpp

@@ -189,10 +189,7 @@ void MachineFunction::init() {
   // Assume the function starts in SSA form with correct liveness.
   Properties.set(MachineFunctionProperties::Property::IsSSA);
   Properties.set(MachineFunctionProperties::Property::TracksLiveness);
-  if (STI->getRegisterInfo())
-    RegInfo = new (Allocator) MachineRegisterInfo(this);
-  else
-    RegInfo = nullptr;
+  RegInfo = new (Allocator) MachineRegisterInfo(this);
 
   MFInfo = nullptr;