cmd/compile: add a test for writebarrier pass with single-block loop

cmd/compile: add a test for writebarrier pass with single-block loop

The old writebarrier implementation fails to handle single-block
loop where a memory Phi value depends on the write barrier store
in the same block. The new implementation (CL 36834) doesn't have
this problem. Add a test to ensure it.

Fix #19067.

Change-Id: Iab13c6817edc12be8a048d18699b4450fa7ed712
Reviewed-on: https://go-review.googlesource.com/36940
Reviewed-by: David Chase <drchase@google.com>