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, 1 deletions

diff --git a/compile.c b/compile.c
index 0dbefd3155..774dadfc79 100644
--- a/compile.c
+++ b/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;
 		}