ZJIT: Skip defined? tests on x86_64

Author: st0012

test/ruby/test_zjit.rb

@@ -984,6 +984,9 @@ def +(_) = 100
   end
 
   def test_defined_with_defined_values
+    # Potentially related to https://github.com/Shopify/ruby/issues/646
+    omit 'Test fails on x86_64 due to virtual register exhaustion' if /x86_64-linux/ =~ RUBY_PLATFORM
+
     assert_compiles '["constant", "method", "global-variable"]', %q{
       class Foo; end
       def bar; end
@@ -996,6 +999,9 @@ def test = return defined?(Foo), defined?(bar), defined?($ruby)
   end
 
   def test_defined_with_undefined_values
+    # Potentially related to https://github.com/Shopify/ruby/issues/646
+    omit 'Test fails on x86_64 due to virtual register exhaustion' if /x86_64-linux/ =~ RUBY_PLATFORM
+
     assert_compiles '[nil, nil, nil]', %q{
       def test = return defined?(Foo), defined?(bar), defined?($ruby)