[release-branch.go1.9] cmd/compile: fix decomposition of 1-element arrays
    
    The offending rule could move the load to a different block,
    which is always a bad idea.
    
    Fixes #22683
    
    Change-Id: I973c88389b2359f734924d9f45c3fb38e166691d
    Reviewed-on: https://go-review.googlesource.com/77331
    Run-TryBot: Andrew Bonventre <andybons@golang.org>
    Reviewed-by: Keith Randall <khr@golang.org>
    Reviewed-by: David Chase <drchase@google.com>