PR go/89368
    compiler: write barrier check nil-check policy tweak
    
    Tweak the recipe for generating writeBarrier loads to insure that the
    dereference expr is marked as not requiring a nil check (not needed
    for gccgo, but needed for gollvm).
    
    Fixes https://gcc.gnu.org/PR89368
    
    Reviewed-on: https://go-review.googlesource.com/c/162904

From-SVN: r268948