cmd/compile/internal/ssa: clear branch likeliness in clobberBlock

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/96075
Reviewed-by: Josh Bleecher Snyder <[email protected]>
Run-TryBot: Philip Hofer <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: philhofer

src/cmd/compile/internal/ssa/branchelim.go

@@ -139,6 +139,7 @@ func clobberBlock(b *Block) {
 	b.Succs = nil
 	b.Aux = nil
 	b.SetControl(nil)
+	b.Likely = BranchUnknown
 	b.Kind = BlockInvalid
 }