cmd/compile: avoid copying all values in nilcheckelim2

[josharian]

nilcheckelim2 cleans up by copying b.Values in a loop, omitting OpUnknowns. However, the common case is that there are no OpUnknowns, in which case we can skip a lot of work. This is a reminder issue to track whether there are any OpUnknowns and if so, skip the copying entirely.

[gopherbot]

Change https://golang.org/cl/65590 mentions this issue: cmd/compile: avoid copying in nilcheckelim2

[gopherbot]

Change https://golang.org/cl/65651 mentions this issue: cmd/compile: avoid copying in nilcheckelim2