RFR: 8255378: [Vector API] Remove redundant vector length check after JDK-8254814 and JDK-8255210

[Jie Fu]

Hi all,

As @iwanowww pointed out [1] that there are redundant vector length checks for reductionI [2] and reductionS [3].
It would be better to remove them.

Testing:
 - jdk/incubator/vector on both AVX512 and AVX256 machines

Thanks.
Best regards,
Jie

[1] https://github.com/openjdk/jdk/pull/791#discussion_r510687005
[2] https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/x86/x86.ad#L4429
[3] https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/x86/x86.ad#L4625

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Commit messages:
 - 8255378: [Vector API] Remove redundant vector length check after JDK-8254814 and JDK-8255210

Changes: https://git.openjdk.java.net/jdk/pull/854/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=854&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255378
  Stats: 4 lines in 1 file changed: 0 ins; 2 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/854.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/854/head:pull/854

PR: https://git.openjdk.java.net/jdk/pull/854