RFR: 8284584: Avoid duplicate node_idx_t definitions

[Vladimir Kozlov]

On Fri, 8 Apr 2022 13:25:16 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> While looking at `Node`, I realized there are two definitions for node_idx_t, another one added by [JDK-8076284](https://bugs.openjdk.java.net/browse/JDK-8076284). These should be de-duplicated to avoid confusion.

I am not sure we want to move C2's specific type to general code.
`node.hpp` includes `compile.hpp` so may be leave definition there only.

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PR: https://git.openjdk.java.net/jdk/pull/8164