RFR: 8284883: JVM crash: guarantee(sect->end() <= sect->limit()) failed: sanity on AVX512 [v2]

[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.

Dean Long has updated the pull request incrementally with three additional commits since the last revision:

 - use loop for large constant sizes passed to clear_mem
 - choose better array size to test more clear_mem paths
 - revert

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8457/files
  - new: https://git.openjdk.java.net/jdk/pull/8457/files/89be65e4..2c6626c5

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8457&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8457&range=00-01

  Stats: 45 lines in 5 files changed: 30 ins; 4 del; 11 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