RFR: 8320209: VectorMaskGen clobbers rflags on x86_64

[Sandhya Viswanathan]

VectorMaskGen clobbers rflags on x86_64 but the corresponding instruct in x86.ad file doesn't list the rflags as being killed.
Adding rFlagsReg cr as being killed in effects for vmask_gen instruct.

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Commit messages:
 - add rflags to kill list for VectorMaskGen

Changes: https://git.openjdk.org/jdk/pull/16680/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16680&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320209
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/16680.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16680/head:pull/16680

PR: https://git.openjdk.org/jdk/pull/16680