RFR: 8265816: Handle new VectorMaskCast node for x86

[Sandhya Viswanathan]

To eliminate unnecessary vector mask conversion during unboxing a
VectorMaskCast node was added as part of JDK-8264104 for aarch64. This patch handles the x86 side of it.

Please review.

Best Regards,
Sandhya

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

Commit messages:
 - 8265816: Handle new VectorMaskCast node for x86

Changes: https://git.openjdk.java.net/jdk/pull/3645/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3645&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265816
  Stats: 12 lines in 1 file changed: 12 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3645.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3645/head:pull/3645

PR: https://git.openjdk.java.net/jdk/pull/3645