Add verification to loop reduction unit tests

[0xdaryl]

The unit tests for loop reduction optimizations that copy memory really only test whether the transformed loop ran for the correct number of iterations. They do not test whether that loop did the correct thing.

For example, https://github.com/eclipse-openj9/openj9/blob/master/test/functional/JIT_Test/src/jit/test/loopReduction/byte2CharMemCpy.java

There should be some verification of the operation performed as well for this to be a truly effective unit test.

In the past, these unit tests were notorious for taking a long time to run and suffered from timeouts in the build. Perhaps that is the reason that validation of the results is skipped. However, the tests themselves could be redesigned to save some time by reducing the number of vector lengths being tested (presently 0 through 15, and then 16 through 2048 in powers of two). Testing that many vector lengths likely isn't providing any value in terms of coverage.

[preveen-stack]

@0xdaryl Is this issue still valid? The link mentioned above is giving 404