RFR: 8272413: Incorrect num of element count calculation for vector cast

[Wang Huang]

Dear all, 
Closed JDK-8265244 has split into two issues : JDK-8268966 and this issue. During this issue, I will fix the mid-end comparsion.
This patch is easy to understand.  It is split from https://github.com/openjdk/jdk/pull/3507. I only fix the mid-end problem because the back-end problem has fixed in JDK-8268966 by @theRealELiu .
Thank you for your review. 

Yours,
WANG Huang

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Commit messages:
 - 8272413: Incorrect num of element count calculation for vector cast

Changes: https://git.openjdk.java.net/jdk/pull/5160/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5160&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272413
  Stats: 3 lines in 1 file changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5160.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5160/head:pull/5160

PR: https://git.openjdk.java.net/jdk/pull/5160