Allows instructions with a From3 field to be used in regopt so
long as From3 represents a constant. This is needed because the
storage-to-storage instructions on s390x place the length of the
data into From3.

Change-Id: I12cd32d4f997baf2fe97937bb7d45bbf716dfcb5
Reviewed-on: https://go-review.googlesource.com/20875Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>