From CL 199979, I noticed that there were some
instructions not covered by the test cases. Added those in this CL.

Additional tests for assembly instructions are also added
based on suggestions made during the review of this CL.

Previously, VSB and VSH are not included in asmz.go, they were also
added in this patch.

Change-Id: I6060a9813b483a161d61ad2240c30eec6de61536
Reviewed-on: https://go-review.googlesource.com/c/go/+/203721Reviewed-by: Michael Munday <mike.munday@ibm.com>