8222670 patch review: prevent downgraded tasks from recompiling

[Liu, Xin]

Hi, hotspot-compiler group,

Could you review this webrev for JDK-8222670?
https://cr.openjdk.java.net/~xliu/8222670/webrev.01/

Inside of our company, we saw some services suffer from recompilation and codecache bloat.
Eg. the attachment was exacted from a real application logs.  It happened randomly and eventually it would fill up the whole codecache.

I wrote a testcase to simulate this problem: Level2RecompilationTest.java
I don’t know how to enqueue a OSR method. I know hard-wired bci=15 sounds stupid.  Arbitrary bci doesn’t work. It may crash in anaalyis.  Any suggest?

This  patch is to detect pre-compiled nmethod when downgrade from level3 to level2.  It will drop the task if  it has been compiled.

Thanks,
--lx

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