RFR: 8276453: Undefined behavior in C1 LIR_OprDesc causes SEGV in fastdebug build [v6]

[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()".

Man Cao has updated the pull request incrementally with two additional commits since the last revision:

 - Use nullOpr() or {} instead of LIR_Opr()
 - Revert the renaming from LIR_OprDesc to LIR_Opr to minimize patch size

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6221/files
  - new: https://git.openjdk.java.net/jdk/pull/6221/files/d881f81d..adaf6d4e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6221&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6221&range=04-05

  Stats: 256 lines in 21 files changed: 14 ins; 0 del; 242 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