cmd/compile: avoid updating capacity if unused

Consider the following code:
```go
func ASCII(b []byte) bool {
	for len(b) >= 8 {
		x := binary.LittleEndian.Uint64(b)
		if x&0x8080808080808080 > 0 {
			return false
		}
		b = b[8:]
	}
	for _, x := range b {
		if x&0x80 > 0 {
			return false
		}
	}
	return true
}
```