RFR: 8341581: Optimize BytecodeHelpers validate slot

[Shaojin Wen]

Use `slot & ~0xFF == 0` instead of `(slot & 0xFF) == slot` to reduce codeSize.

The following is a comparison of the bytecode before and after the modification. It can be seen that using `slot & ~0xFF == 0` will reduce one iload operation.


// (slot & 0xFF) == slot
56: iload_0
57: sipush        255
60: iand
61: iload_0
62: if_icmpne     71

// slot & ~0xFF == 0
56: iload_0
57: sipush        -256
60: iand
61: ifne          70

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

Commit messages:
 - optimize validate slot
 - optimize validate slot

Changes: https://git.openjdk.org/jdk/pull/21367/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21367&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341581
  Stats: 26 lines in 1 file changed: 0 ins; 0 del; 26 mod
  Patch: https://git.openjdk.org/jdk/pull/21367.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21367/head:pull/21367

PR: https://git.openjdk.org/jdk/pull/21367