RFR: 8313564: Fix -Wconversion warnings in classfile code [v3]

[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.

Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:

  David's suggestions.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/15111/files
  - new: https://git.openjdk.org/jdk/pull/15111/files/0e97905c..864a80f7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15111&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15111&range=01-02

  Stats: 14 lines in 4 files changed: 0 ins; 0 del; 14 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