Add more GC guards

bootstraptest/test_ractor.rb

@@ -1532,7 +1532,7 @@ assert_equal "ok", %q{
 
   1_000.times { idle_worker, tmp_reporter = Ractor.select(*workers) }
   "ok"
-}
+} unless ENV['RUN_OPTS'] =~ /mjit/ # flaky
 
 assert_equal "ok", %q{
   def foo(*); ->{ super }; end

lib/ruby_vm/mjit/compiler.rb

@@ -289,7 +289,7 @@ module RubyVM::MJIT
     end
 
     def mjit_blocks(iseq)
-      # Tolerate GC on any ISEQ
+      # Guard against ISEQ GC at random moments
       if C.imemo_type(iseq) != C.imemo_iseq
         return Hash.new { |h, k| h[k] = {} }
       end

lib/ruby_vm/mjit/exit_compiler.rb

@@ -82,7 +82,10 @@ module RubyVM::MJIT
     # @param target0_p [TrueClass,FalseClass]
     def compile_branch_stub(ctx, asm, branch_stub, target0_p)
       # Call rb_mjit_branch_stub_hit
-      asm.comment("branch stub hit: #{branch_stub.iseq.body.location.label}@#{C.rb_iseq_path(branch_stub.iseq)}:#{iseq_lineno(branch_stub.iseq, target0_p ? branch_stub.target0.pc : branch_stub.target1.pc)}")
+      iseq = branch_stub.iseq
+      if C.imemo_type(iseq) != C.imemo_iseq # Guard against ISEQ GC at random moments
+        asm.comment("branch stub hit: #{iseq.body.location.label}@#{C.rb_iseq_path(iseq)}:#{iseq_lineno(iseq, target0_p ? branch_stub.target0.pc : branch_stub.target1.pc)}")
+      end
       asm.mov(:rdi, to_value(branch_stub))
       asm.mov(:esi, ctx.sp_offset)
       asm.mov(:edx, target0_p ? 1 : 0)