Write barrier unsafe-point analysis needs to flow through
OpARM64MOVWUload in c-shared mode.

Change-Id: I4f06f54d9e74a739a1b4fcb9ab0a1ae9b7b88a95
Reviewed-on: https://go-review.googlesource.com/114077
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: David Chase <drchase@google.com>