compiler: compare slice against nil

testdata/slice.txt

@@ -1,9 +1,11 @@
+foo is nil? false false
 foo: len=4 cap=4 data: 1 2 4 5
 bar: len=3 cap=5 data: 0 0 0
 foo[1:2]: len=1 cap=3 data: 2
 sum foo: 12
 copy foo -> bar: 3
 bar: len=3 cap=5 data: 1 2 4
+slice is nil? true true
 grow: len=0 cap=0 data:
 grow: len=1 cap=1 data: 42
 grow: len=3 cap=4 data: 42 -1 -2