RFR: 8276453: Undefined behavior in C1 LIR_OprDesc causes SEGV in fastdebug build

[Man Cao]

Hi all,

Could anyone provide some feedback on this bug fix and refactoring change? See https://bugs.openjdk.java.net/browse/JDK-8276453 for more details.
If the direction of this change looks good, we can proceed removing the "UGLY HACK" in c1_LIR.hpp and refactor occurrences of "opr->fn()" to "opr.fn()".

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Commit messages:
 - Add _value field and rename LIR_OprDesc to LIR_Opr

Changes: https://git.openjdk.java.net/jdk/pull/6221/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6221&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276453
  Stats: 287 lines in 25 files changed: 23 ins; 16 del; 248 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6221.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6221/head:pull/6221

PR: https://git.openjdk.java.net/jdk/pull/6221