RFR: 8283451: C2: assert(_base == Long) failed: Not a Long

[Roland Westrelin]

The code in PhaseIterGVN::no_dependent_zero_check() tests for a top
input by checking if is_top() is true but in this case the node has
type top but is not yet transformed to top (after CCP).

-------------

Commit messages:
 - whitespaces
 - test
 - fix

Changes: https://git.openjdk.java.net/jdk/pull/7902/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7902&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283451
  Stats: 78 lines in 2 files changed: 76 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7902.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7902/head:pull/7902

PR: https://git.openjdk.java.net/jdk/pull/7902