[dev.simd] simd: modify test generation to make it more flexible

[dev.simd] simd: modify test generation to make it more flexible

This is to support conversions, which are not T -> T.

Change-Id: I323887b116eee8133770a899ed82363bba38a9c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/689717
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>