[dev.ssa] cmd/compile: Handle ONOT in conditionals as well

Might as well, for a && !(b && c) and the like.

Change-Id: I2548b6e6ee5870e074bcef6edd56a7db6e81d70f
Reviewed-on: https://go-review.googlesource.com/16600
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>

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

@@ -2026,6 +2026,11 @@ func (s *state) condBranch(cond *Node, yes, no *ssa.Block, likely int8) {
 		// If likely==1, then we don't have enough info to decide
 		// the likelihood of the first branch.
 	}
+	if cond.Op == ONOT {
+		s.stmtList(cond.Ninit)
+		s.condBranch(cond.Left, no, yes, -likely)
+		return
+	}
 	c := s.expr(cond)
 	b := s.endBlock()
 	b.Kind = ssa.BlockIf