RFR: 8311847: Fix -Wconversion for assembler.hpp emit_int8, 16 callers

[Coleen Phillimore]

Please review changes to fix -Wconversion warnings that come from assembler_<cpu>.cpp by adding narrow_casts to the emit_int8,16,24, and 32 functions.  And some other fixups with checked_cast.

Ran tier1 on Oracle platforms, and tier1-4 on linux-x64-debug, linux-aarch64-debug, windows-x64-debug.

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Commit messages:
 - Fix -Wconversion for assembler.hpp emit_int8,16 callers

Changes: https://git.openjdk.org/jdk/pull/14822/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14822&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311847
  Stats: 47 lines in 4 files changed: 20 ins; 1 del; 26 mod
  Patch: https://git.openjdk.org/jdk/pull/14822.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14822/head:pull/14822

PR: https://git.openjdk.org/jdk/pull/14822