Compiling without optimizations (-N) can result in write barrier
blocks that have been optimized away but not actually pruned from the
block set. Fix unsafe-point analysis to recognize and ignore these.

For #24543.

Change-Id: I2ca86fb1a0346214ec71d7d6c17b6a121857b01d
Reviewed-on: https://go-review.googlesource.com/114076
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>