RFR: 8300865: C2: product reduction in ProdRed_Double is not vectorized

[Sandhya Viswanathan]

This PR fixes the problem with double reduction on x86_64. 

In the test compiler.loopopts.superword.ProdRed_Double, the product reduction loop in prodReductionImplement() was not getting vectorized when run as follows:
jtreg -XX:CompileCommand=PrintAssembly,compiler.loopopts.superword.ProdRed_Double::prodReductionImplement compiler/loopopts/superword/ProdRed_Double.java
The print assembly generated in the pid-xxx.log output in JTwork/scratch directory was not showing any vector_reduction_double node.

This was happening as the ReductionNode::implemented was passed a vector size of one element. For the vector reduction implemented we need to check with at least vector size of two elements.

With this PR the vector_reduction_double node is generated.

Please review.

Best Regards,
Sandhya

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

Commit messages:
 - Update IR test
 - 8300865: C2: product reduction in ProdRed_Double is not vectorized

Changes: https://git.openjdk.org/jdk/pull/14065/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14065&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300865
  Stats: 9 lines in 2 files changed: 6 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14065.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14065/head:pull/14065

PR: https://git.openjdk.org/jdk/pull/14065