RFR: 8256203: Simplify RegMask::Empty

[Claes Redestad]

- Simplify RegMask::Empty to use default constructor.
- Add missing validation in the empty constructor.

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Commit messages:
 - Simplify RegMask::Empty

Changes: https://git.openjdk.java.net/jdk/pull/1167/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1167&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256203
  Stats: 9 lines in 2 files changed: 2 ins; 5 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1167.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1167/head:pull/1167

PR: https://git.openjdk.java.net/jdk/pull/1167