RFR: 8277508: need to check has_predicated_vectors before calling scalable_predicate_reg_slots

[Yadong Wang]

Hi, Team,
A separate set of predicate registers is not mandatory for an implementation of scalable vectors. It will cause a failure in some platform which supports scalable vectors without explicit predicated registers, like riscv. All code about RegVectMask should be covered by has_predicated_vectors here in Matcher::init_first_stack_mask().

Yadong

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Commit messages:
 - 8277508: missed to check has_predicated_vectors before calling scalable_predicate_reg_slots

Changes: https://git.openjdk.java.net/jdk/pull/6492/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6492&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277508
  Stats: 13 lines in 1 file changed: 2 ins; 0 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6492.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6492/head:pull/6492

PR: https://git.openjdk.java.net/jdk/pull/6492