RFR: 8313564: Fix -Wconversion warnings in classfile code

[Coleen Phillimore]

This patch fixes various -Wconversion warnings in classfile code.  I broke the change into commits so the changes are easier to see.
Tested with tier1-4.

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Commit messages:
 - unset Wconversion
 - Fix compactHashtable.cpp
 - Fix loaderConstraints.cpp, altHashing.cpp, classLoaderExt.cpp
 - Fix fieldLayoutBuilder.cpp by fixing Contended command line parameter.
 - Fix klassFactory.cpp
 - Fix systemDictionaryShared.cpp
 - Fix classLoader.cpp
 - Fix vmIntrinsics.cpp
 - Fix -Wconversion warnings in classfile code.

Changes: https://git.openjdk.org/jdk/pull/15111/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15111&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313564
  Stats: 47 lines in 17 files changed: 2 ins; 0 del; 45 mod
  Patch: https://git.openjdk.org/jdk/pull/15111.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15111/head:pull/15111

PR: https://git.openjdk.org/jdk/pull/15111