RFR: 8305690: [X86] Do not emit two REX prefixes in Assembler::prefix

[Guoxiong Li]

Hi all,

This patch prevents `Assembler::prefix` from emitting two `REX prefixes`. The current code in mainline works well because the corresponding code path is not triggered.

Thanks for the review.

Best Regards,
-- Guoxiong

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Commit messages:
 - 8305690: [X86] Do not emit two REX prefixes in Assembler::prefix

Changes: https://git.openjdk.org/jdk/pull/13369/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13369&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305690
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13369.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13369/head:pull/13369

PR: https://git.openjdk.org/jdk/pull/13369