compiler: avoid write barrier for a[i] = a[i][:v]
compiler: avoid write barrier for a[i] = a[i][:v]

This avoids generating a write barrier for code that appears in the
Go1.14beta1 runtime package in (*pageAlloc).sysGrow:
    s.summary[l] = s.summary[l][:needIdxLimit]

Change-Id: If0c7c76eabbdd68c28e5e9e9233af013144f34f2
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/213558
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend