RFR: 8305203: Simplify trimming operation in Region::Ideal

[Xin Liu]

This patch improves how Region::Ideal trims unreachable paths. 

1. Don't restart from beginning. Trimming doesn't change the DU-chain.
2. Replace DFIterator with DFIterator_Fast. The later is a raw pointer in release build.
3. Don't call add_users_to_worklist(this) repeatly.
4. Reduce its strength from add_users_to_worklist to
   add_users_to_worklist0 because RegionNode has no special logic.

This patch also includes a cosmetic change: rename n to 'use' inside of the loop.
Otherwise, we would overshadow Node* n = in(i). Nothing wrong but harder to read.

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Commit messages:
 - 8305203: Simplify trimming operation in Region::Ideal

Changes: https://git.openjdk.org/jdk/pull/13238/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13238&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305203
  Stats: 24 lines in 1 file changed: 4 ins; 9 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/13238.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13238/head:pull/13238

PR: https://git.openjdk.org/jdk/pull/13238