bare_instructions.rb: use Hash#fetch to read attr

to raise descriptive KeyError instead of NoMethodError in case these
attrs are accidentally removed.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64685 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/ruby_vm/models/bare_instructions.rb

@@ -102,11 +102,11 @@ class RubyVM::BareInstructions
   end
 
   def handles_sp?
-    /\b(false|0)\b/ !~ @attrs['handles_sp'].expr.expr
+    /\b(false|0)\b/ !~ @attrs.fetch('handles_sp').expr.expr
   end
 
   def always_leaf?
-    @attrs['leaf'].expr.expr == 'true;'
+    @attrs.fetch('leaf').expr.expr == 'true;'
   end
 
   def complicated_return_values?