[Bug #17880] Set leaf false on opt_setinlinecache (#4565)

This change fixes the bug described in https://bugs.ruby-lang.org/issues/17880.

Checking `ractor_shareable_p` will cause the method to call back into
Ruby. Anything calling this method can't be a leaf instruction,
otherwise it could crash. By adding `attr bool leaf = false` we no
longer crash because it marks the function as not a leaf.

Here's a simplified reproduction script:

```ruby
require "set"

class Id
  attr_reader :db_id
  def initialize(db_id)
    @db_id = db_id
  end

  def ==(other)
    other.class == self.class && other.db_id == db_id
  end
  alias_method :eql?, :==

  def hash
    10
  end

  def <=>(other)
    db_id <=> other.db_id if other.is_a?(self.class)
  end
end

class Namespace
  IDS = Set[
    Id.new(1).freeze,
    Id.new(2).freeze,
    Id.new(3).freeze,
    Id.new(4).freeze,
  ].freeze

  class << self
    def test?(id)
      IDS.include?(id)
    end
  end
end

p Namespace.test?(Id.new(1))
p Namespace.test?(Id.new(5))
```

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

insns.def

@@ -1023,6 +1023,7 @@ opt_setinlinecache
 (IC ic)
 (VALUE val)
 (VALUE val)
+// attr bool leaf = false;
 {
     vm_ic_update(GET_ISEQ(), ic, val, GET_EP());
 }

test/ruby/test_insns_leaf.rb

@@ -0,0 +1,46 @@
+# frozen_string_literal: false
+require 'test/unit'
+
+class TestInsnsLeaf < Test::Unit::TestCase
+  require "set"
+
+  class Id
+    attr_reader :db_id
+    def initialize(db_id)
+      @db_id = db_id
+    end
+
+    def ==(other)
+      other.class == self.class && other.db_id == db_id
+    end
+    alias_method :eql?, :==
+
+    def hash
+      10
+    end
+
+    def <=>(other)
+      db_id <=> other.db_id if other.is_a?(self.class)
+    end
+  end
+
+  class Namespace
+    IDS = Set[
+      Id.new(1).freeze,
+      Id.new(2).freeze,
+      Id.new(3).freeze,
+      Id.new(4).freeze,
+    ].freeze
+
+    class << self
+      def test?(id)
+	IDS.include?(id)
+      end
+    end
+  end
+
+  def test_insns_leaf
+    assert Namespace.test?(Id.new(1)), "IDS should include 1"
+    assert !Namespace.test?(Id.new(5)), "IDS should not include 5"
+  end
+end