[release-branch.go1.10] cmd/compile: reset branch prediction when deleting a branch

When we go from a branch block to a plain block, reset the
branch prediction bit. Downstream passes asssume that if the
branch prediction is set, then the block has 2 successors.

Fixes #23504
Fixes #26851

Change-Id: I2898ec002228b2e34fe80ce420c6939201c0a5aa
Reviewed-on: https://go-review.googlesource.com/88955


Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
(cherry picked from commit 4313d776)
Reviewed-on: https://go-review.googlesource.com/128855


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>