Revert "Add a specialized instruction for `.nil?` calls"

This reverts commit 9faef3113f.

It seemed to cause a failure on macOS Mojave, though I'm unsure how.
https://rubyci.org/logs/rubyci.s3.amazonaws.com/osx1014/ruby-master/log/20190802T034503Z.fail.html.gz

This tentative revert is to check if the issue is actually caused by the
change or not.

benchmark/nil_p.yml

@@ -1,9 +0,0 @@
-prelude: |
-  class Niller; def nil?; true; end; end
-  xnil, notnil = nil, Object.new
-  niller = Niller.new
-benchmark:
-  - xnil.nil?
-  - notnil.nil?
-  - niller.nil?
-loop_count: 10000000

compile.c

@@ -3255,7 +3255,6 @@ iseq_specialized_instruction(rb_iseq_t *iseq, INSN *iobj)
 		  case idLength: SP_INSN(length); return COMPILE_OK;
 		  case idSize:	 SP_INSN(size);	  return COMPILE_OK;
 		  case idEmptyP: SP_INSN(empty_p);return COMPILE_OK;
-                  case idNilP:   SP_INSN(nil_p);  return COMPILE_OK;
 		  case idSucc:	 SP_INSN(succ);	  return COMPILE_OK;
 		  case idNot:	 SP_INSN(not);	  return COMPILE_OK;
 		}

defs/id.def

@@ -3,7 +3,6 @@ firstline, predefined = __LINE__+1, %[\
   max
   min
   freeze
-  nil?
   inspect
   intern
   object_id

insns.def

@@ -808,20 +808,6 @@ opt_str_freeze
     }
 }
 
-/* optimized nil? */
-DEFINE_INSN
-opt_nil_p
-(CALL_INFO ci, CALL_CACHE cc)
-(VALUE recv)
-(VALUE val)
-{
-    val = vm_opt_nil_p(ci, cc, recv);
-
-    if (val == Qundef) {
-        CALL_SIMPLE_METHOD();
-    }
-}
-
 DEFINE_INSN
 opt_str_uminus
 (VALUE str, CALL_INFO ci, CALL_CACHE cc)

object.c

@@ -1687,7 +1687,7 @@ rb_true(VALUE obj)
  */
 
 
-MJIT_FUNC_EXPORTED VALUE
+static VALUE
 rb_false(VALUE obj)
 {
     return Qfalse;

vm.c

@@ -1656,7 +1656,6 @@ vm_init_redefined_flag(void)
     OP(Call, CALL), (C(Proc));
     OP(And, AND), (C(Integer));
     OP(Or, OR), (C(Integer));
-    OP(NilP, NIL_P), (C(NilClass));
 #undef C
 #undef OP
 }

vm_core.h

@@ -550,7 +550,6 @@ enum ruby_basic_operators {
     BOP_LENGTH,
     BOP_SIZE,
     BOP_EMPTY_P,
-    BOP_NIL_P,
     BOP_SUCC,
     BOP_GT,
     BOP_GE,

vm_insnhelper.c

@@ -4228,26 +4228,6 @@ vm_opt_empty_p(VALUE recv)
     }
 }
 
-VALUE rb_false(VALUE obj);
-
-static VALUE
-vm_opt_nil_p(CALL_INFO ci, CALL_CACHE cc, VALUE recv)
-{
-    if (recv == Qnil) {
-        if (BASIC_OP_UNREDEFINED_P(BOP_NIL_P, NIL_REDEFINED_OP_FLAG)) {
-            return Qtrue;
-        } else {
-            return Qundef;
-        }
-    } else {
-        if (vm_method_cfunc_is(ci, cc, recv, rb_false)) {
-            return Qfalse;
-        } else {
-            return Qundef;
-        }
-    }
-}
-
 static VALUE
 fix_succ(VALUE x)
 {