RFR: 8265322: C2: Simplify control inputs for BarrierSetC2::obj_allocate

[Yi Yang]

This PR simplifies control inputs for BarrierSetC2::obj_allocate. In most cases, it doesn't change anything since `toobig_false` is equivalent to `ctrl`. In rare case, `toobig_false` is created for Unsafe.allocateInstance while instance size is not statically known, `ctrl` would become control input of IfNode whose projects are `toobig_false` and `toobig_true`, old eden_end and old_eden_top can simply accept `toobig_false` as their control input rather than `ctrl`.

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Commit messages:
 - simplify

Changes: https://git.openjdk.java.net/jdk/pull/3529/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3529&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265322
  Stats: 5 lines in 3 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3529.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3529/head:pull/3529

PR: https://git.openjdk.java.net/jdk/pull/3529