RFR: 8264958: C2 compilation fails with assert "n is later than its clone"

[Roland Westrelin]

JDK-8229483 added logic to hoist a load that would wrongly end up in
an outer strip mined loop. That logic checks that it's legal to do so
with:

is_dominator(n_ctrl, x_head)

but that test passes for n_ctrl == x_head when it's not legal to hoist
the load i.e. the test we want is for strict domination. The fix I
propose is to add an explicit check for that case.

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Commit messages:
 - test
 - fix

Changes: https://git.openjdk.java.net/jdk/pull/3539/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3539&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264958
  Stats: 58 lines in 2 files changed: 57 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3539.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3539/head:pull/3539

PR: https://git.openjdk.java.net/jdk/pull/3539