I assumed somehow this check was not needed, but it did need.
By canceling this instead of just warning here, we didn't lose the
current performance so much.

test_jit.rb: test the case that reproduces SEGV by that.

TestGemStreamUI.rb: delete. This test on --jit-wait is fixed.

=== Optcarrot Benchmark ===

$ benchmark-driver benchmark.yml --rbenv 'before::before --disable-gems --jit;after::after --disable-gems --jit' -v --repeat-count 24
before: ruby 2.6.0dev (2018-10-24 trunk 65355) +JIT [x86_64-linux]
after: ruby 2.6.0dev (2018-10-24 trunk 65355) +JIT [x86_64-linux]
last_commit=_mjit_compile_ivar.erb: cancel on undefined ivar
Calculating -------------------------------------
                             before       after
Optcarrot Lan_Master.nes     85.344      84.849 fps

Comparison:
             Optcarrot Lan_Master.nes
                  before:        85.3 fps
                   after:        84.8 fps - 1.01x  slower

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65356 b2dd03c8-39d4-4d8f-98ff-823fe69b080e