cmd/compile: soften test for 74788

cmd/compile: soften test for 74788

We now (as of CL 678620) use float registers other than X0 for copying.

Change-Id: Ifdecd5df7519663742eed0f292c98453754d4b25
Reviewed-on: https://go-review.googlesource.com/c/go/+/695275
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>