[TargetInstrInfo][NFC] Don't restrict isAddImmediate description to p…

…hysical registers (llvm#72357)

None of the in-tree implementations have different behaviour for
physical vs virtual registers, and it seems would work equally well if
used with virtual registers. As such, perhaps it's simplest to just drop
that part of the doc comment.

llvm/include/llvm/CodeGen/TargetInstrInfo.h

@@ -1060,9 +1060,9 @@ class TargetInstrInfo : public MCInstrInfo {
   }
 
   /// If the specific machine instruction is an instruction that adds an
-  /// immediate value and a physical register, and stores the result in
-  /// the given physical register \c Reg, return a pair of the source
-  /// register and the offset which has been added.
+  /// immediate value and a register, and stores the result in the given
+  /// register \c Reg, return a pair of the source register and the offset
+  /// which has been added.
   virtual std::optional<RegImmPair> isAddImmediate(const MachineInstr &MI,
                                                    Register Reg) const {
     return std::nullopt;