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

[Aleksey Shipilev]

On Sun, 14 Jul 2024 12:00:22 GMT, Kim Barrett <kbarrett 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

Marked as reviewed by shade (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/20174#pullrequestreview-2178089079