RFR: 8310727: C2: *_of() methods in PhaseIterGVN should use uint for the node index

[Daniel Lundén]

This changeset addresses the below.
> Some *_of() methods in PhaseIterGVN like replace_input_of() use int instead of uint for the node index. We should change that.

### Testing (on all Oracle-supported platforms)
- `tier1` 
- HotSpot parts of `tier2` and `tier3`

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Commit messages:
 - Change int to uint

Changes: https://git.openjdk.org/jdk/pull/15946/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15946&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310727
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/15946.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15946/head:pull/15946

PR: https://git.openjdk.org/jdk/pull/15946