RFR: 8257624: C2: PhaseMacroExpand::eliminate_macro_nodes() crashes on out-of-bounds access into macro node array

[Vladimir Ivanov]

Elimination of a single macro node may trigger removal of some other macro nodes (e.g., see `PhaseMacroExpand::process_users_of_allocation()`  which can eliminate `AllocateCopy` if it is a user of `Allocate` node being scalar replaced). But `PhaseMacroExpand::eliminate_macro_nodes()` doesn't take it into account: it iterates over the array backwards one by one and if it is unfortunate to eliminate multiple elements when it is at the very end of the array, it crashes on an out-of-bounds access.

The fix is to adjust current position on every iteration. If there are multiple elements removed, current position can point at an element which is already processed, but that's benign.

Testing:
- [x] failing tests
- [x] hs-tier-1-6 w/ -XX:+AlwaysIncrementalInline

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Commit messages:
 - 8257624: C2: PhaseMacroExpand::eliminate_macro_nodes() crashes on out-of-bounds access into macro node array

Changes: https://git.openjdk.java.net/jdk/pull/1572/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1572&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257624
  Stats: 8 lines in 1 file changed: 6 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1572.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1572/head:pull/1572

PR: https://git.openjdk.java.net/jdk/pull/1572