[jdk17u-dev] 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.

Low risk:
It is a corner case which is not triggered by the current code.
In order to avoid bugs in the future, it is good to backport it.

Thanks for taking the time to review.

Best Regards,
-- Guoxiong

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Commit messages:
 - Backport 49726ee3a95023a912aacad0e3714eae146eed21

Changes: https://git.openjdk.org/jdk17u-dev/pull/1334/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1334&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/jdk17u-dev/pull/1334.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/1334/head:pull/1334

PR: https://git.openjdk.org/jdk17u-dev/pull/1334