[dev.simd] simd: add testing hooks for size-changing conversions

[dev.simd] simd: add testing hooks for size-changing conversions

and adds some tests of size-changing conversions.

IMO the template naming conventions in genfiles
are getting grubby, and I plan to change them in
an immediately following CL.

Change-Id: I4a72e8a8c9e9806fab60570dff4c87a754e427c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/697456
Commit-Queue: David Chase <drchase@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
TryBot-Bypass: David Chase <drchase@google.com>