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

[Coleen Phillimore]

This is essentially the same as PR https://github.com/openjdk/jdk/pull/14396 only without the asm/assembler.hpp changes for emit_int8, etc.  The casts and type changes here resolve header file -Wconversion warnings but not the 941 in assembler_x86.cpp.
Tested with tier 1-7, and tier1 on 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/14584/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14584&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309685
  Stats: 61 lines in 20 files changed: 0 ins; 0 del; 61 mod
  Patch: https://git.openjdk.org/jdk/pull/14584.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14584/head:pull/14584

PR: https://git.openjdk.org/jdk/pull/14584