RFR: 8264360: Loop strip mining verification fails with "should be on the backedge"

[Roland Westrelin]

The assert checks that loads that are found as uses of a loop back
edge have the back edge as control input. In this case, because that
code is executed during final graph reshaping (CastPPs are in the
process of being eliminated and memory operations inherit their
control inputs as precedence edges), the edge between a load and the
back edge is a precedence edge. Relaxing the assert is all that is
required.

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

Commit messages:
 - test
 - fix

Changes: https://git.openjdk.java.net/jdk/pull/3245/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3245&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264360
  Stats: 73 lines in 2 files changed: 72 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3245.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3245/head:pull/3245

PR: https://git.openjdk.java.net/jdk/pull/3245