RFR: 8309685: Fix -Wconversion warnings in assembler and register code

[Coleen Phillimore]

This change widens some int arguments, narrows some int returns and adds some casts and checked_casts where the compiler was doing the implicit conversion.  I maintained existing types - ie if the parameter was short, I used a cast to short.  Also int is used in places that might be better served as unsigned int but I didn't fix that because it would be too large and risky.  The registers encode an offset in an array, so it's safe to checked_cast<> to get their encoding. This fix is limited so that the types changed and casts added are intentional.
See CR for counts of -Wconversion warnings this resolves.
Tested with tier1-7, also tested with tier1 on all Oracle supported platforms.

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Commit messages:
 - 8309685: Fix -Wconversion warnings in assembler and register code

Changes: https://git.openjdk.org/jdk/pull/14396/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14396&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309685
  Stats: 66 lines in 19 files changed: 2 ins; 0 del; 64 mod
  Patch: https://git.openjdk.org/jdk/pull/14396.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14396/head:pull/14396

PR: https://git.openjdk.org/jdk/pull/14396