cmd/compile: confusion between "don't need bounds checks" and "don't need nil checks"

[josharian]

CL 228106 introduced a bug. CL 228677 fixed it. See those two CLs for details. This issue exists to obtain an issue number for a test to ensure that it remains fixed.

[gopherbot]

Change https://golang.org/cl/228698 mentions this issue: test: add test for nil check / bounds check compiler confusion

[gopherbot]

Change https://golang.org/cl/228787 mentions this issue: cmd/compile: clarify Node.NonNil and Node.Bounded