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

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


Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>