RFR: 8275337 C1: assert(false) failed: live_in set of first block must be empty

[Igor Veresov]

When determining if an instruction is loop invariant we should check its dominance instead of just comparing the dominator tree depths.

Tier1-7 tests are clean.

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Commit messages:
 - C1 RCE: Tighten the loop invariant check

Changes: https://git.openjdk.java.net/jdk/pull/7276/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7276&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275337
  Stats: 71 lines in 2 files changed: 70 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7276.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7276/head:pull/7276

PR: https://git.openjdk.java.net/jdk/pull/7276