RFR: 8276453: Undefined behavior in C1 LIR_OprDesc causes SEGV in fastdebug build [v6]

[Dean Long]

On Thu, 11 Nov 2021 23:53:48 GMT, Man Cao <manc at openjdk.org> wrote:

>> Man Cao has updated the pull request incrementally with two additional commits since the last revision:
>> 
>>  - Use nullOpr() or {} instead of LIR_Opr()
>>  - Revert the renaming from LIR_OprDesc to LIR_Opr to minimize patch size
>
> I uploaded new result with `-XX:TieredStopAtLevel=3 -XX:CICompilerCount=1` to https://bugs.openjdk.java.net/browse/JDK-8276453. `java.ci.totalTime` is still unchanged.
> 
> Also added `_value != 0` in assertion.

@caoman This looks good.

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PR: https://git.openjdk.java.net/jdk/pull/6221