[CodeGen] Return ArrayRef from TargetRegisterClass::getRegisters. NFCI. (#80411)

This will allow future patches to use indexing and methods like
drop_front on the result.

Author: jayfoad

llvm/include/llvm/CodeGen/TargetRegisterInfo.h

@@ -81,9 +81,8 @@ class TargetRegisterClass {
   /// Return the number of registers in this class.
   unsigned getNumRegs() const { return MC->getNumRegs(); }
 
-  iterator_range<SmallVectorImpl<MCPhysReg>::const_iterator>
-  getRegisters() const {
-    return make_range(MC->begin(), MC->end());
+  ArrayRef<MCPhysReg> getRegisters() const {
+    return ArrayRef(begin(), getNumRegs());
   }
 
   /// Return the specified register in the class.
@@ -203,7 +202,7 @@ class TargetRegisterClass {
   ///
   /// By default, this method returns all registers in the class.
   ArrayRef<MCPhysReg> getRawAllocationOrder(const MachineFunction &MF) const {
-    return OrderFunc ? OrderFunc(MF) : ArrayRef(begin(), getNumRegs());
+    return OrderFunc ? OrderFunc(MF) : getRegisters();
   }
 
   /// Returns the combination of all lane masks of register in this class.