RFR: 8284883: JVM crash: guarantee(sect->end() <= sect->limit()) failed: sanity on AVX512

[Dean Long]

This fix prevents overflowing the C2 scratch buffer for large ClearArray operations.  I also noticed that when IdealizeClearArrayNode is turned off, the "is_large" flag on the ClearArray node was not set correctly, so I fixed that too.

I could use some help testing the x86_32 change.

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Commit messages:
 - prevent overflowing scratch buffer
 - always set ClearArray _is_large flag correctly
 - reproducer test

Changes: https://git.openjdk.java.net/jdk/pull/8457/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8457&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284883
  Stats: 58 lines in 4 files changed: 55 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8457.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8457/head:pull/8457

PR: https://git.openjdk.java.net/jdk/pull/8457