cmd/compile: unneeded bounds checks

[marigonzes]

What version of Go are you using (go version)?

$ go version
go version devel +2197321 Tue Oct 8 23:53:55 2019 +0000 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What did you do?

I compiled the following functions to check if there were any bounds checks: https://godbolt.org/z/RSrNwR.

What did you expect to see?

I expected to see no bounds checks.

What did you see instead?

Instead, I see 7 different bounds checks generated by the Go compiler (highlighted in the site above).
I talked with @zdjones about this and it appears like all these bounds checks can be removed.