[dev.ssa] cmd/compile/ssa: Replace less-or-equal with equal in len comparison with zero

Since the spec guarantees than 0 <= len always:

https://golang.org/ref/spec#Length_and_capacity

replace len(...) <= 0 check with len(...) == 0 check

Change-Id: I5517a9cb6b190f0b1ee314a67487477435f3b409
Reviewed-on: https://go-review.googlesource.com/12034
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -48,7 +48,7 @@ func (bp *blockPool) newBlock() *Block {
 	bp.mu.Lock()
 	defer bp.mu.Unlock()
 
-	if len(bp.blocks) <= 0 {
+	if len(bp.blocks) == 0 {
 		bp.blocks = make([]Block, blockSize, blockSize)
 	}