RFR: 8309685: Fix -Wconversion warnings in assembler and register code [v4]

[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.

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

  First fix math.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14396/files
  - new: https://git.openjdk.org/jdk/pull/14396/files/f74390f6..7812932b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14396&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14396&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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