[IR] Remove arg_operands and getNumArgOperands (NFC)

The last uses were removed on Oct 8, 2021 in commit
46ef2e0.

This is a relanding of b2ee408.

Author: kazutakahirata

llvm/include/llvm/IR/InstrTypes.h

@@ -1325,16 +1325,6 @@ class CallBase : public Instruction {
   bool arg_empty() const { return arg_end() == arg_begin(); }
   unsigned arg_size() const { return arg_end() - arg_begin(); }
 
-  // Legacy API names that duplicate the above and will be removed once users
-  // are migrated.
-  iterator_range<User::op_iterator> arg_operands() {
-    return make_range(arg_begin(), arg_end());
-  }
-  iterator_range<User::const_op_iterator> arg_operands() const {
-    return make_range(arg_begin(), arg_end());
-  }
-  unsigned getNumArgOperands() const { return arg_size(); }
-
   Value *getArgOperand(unsigned i) const {
     assert(i < arg_size() && "Out of bounds!");
     return getOperand(i);