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

This reverts commit 9faef3113fb4331524b81ba73005ba13fa0ef6c6.

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.

1 files changed, 0 insertions, 14 deletions

diff --git a/insns.def b/insns.def
index 7c93af6e4d..c971026cf6 100644
--- a/insns.def
+++ b/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)