RFR: 8336349: Fix more simple -Wzero-as-null-pointer-constant warnings in C2 code

[Kim Barrett]

On Mon, 15 Jul 2024 14:55:25 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> Please review this change that replaces some uses of literal 0 as a null
>> pointer constant in C2 code to instead use nullptr.
>> 
>> Testing: mach5 tier1
>
> Looks good.

Thanks for reviews @vnkozlov and @shipilev

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PR Comment: https://git.openjdk.org/jdk/pull/20174#issuecomment-2228842005