RFR: JDK-8290822: C2: assert in PhaseIdealLoop::do_unroll() is subject to undefined behavior

[Tobias Holenstein]

The following assert had undefined behavior (UB) because of _signed_ integer underflow/overflow: 

    assert(stride_con > 0 && ((limit_type->_hi - stride_con) < limit_type->_hi) || 
           stride_con < 0 && ((limit_type->_lo - stride_con) > limit_type->_lo), 
           "sanity"); 


# Solution 
The fix is to check for underflow/overflow (the purpose of the assert) without actually underflowing/overflowing.

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Commit messages:
 - assert should check that NO overflow occurs
 - JDK-8290822: C2: assert in PhaseIdealLoop::do_unroll() is subject to undefined behavior

Changes: https://git.openjdk.org/jdk/pull/12576/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12576&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290822
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12576.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12576/head:pull/12576

PR: https://git.openjdk.org/jdk/pull/12576