The branchelim pass makes some blocks unreachable, but does not
remove them from Func.Values. Consequently, ssacheck complains
when it finds a block with a non-zero likeliness value but no
successors.

Fixes #24014

Change-Id: I2dcf1d8f4e769a2f363508dab3b11198ead336b6
Reviewed-on: https://go-review.googlesource.com/96075Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Philip Hofer <phofer@umich.edu>
TryBot-Result: Gobot Gobot <gobot@golang.org>