cmd/compile: function compiled without bounds checking and -1 index access [1.11 backport]

[gopherbot]

@FiloSottile requested issue #27251 to be considered for backport to the next 1.11 minor release.

@gopherbot please file this for backport against 1.11. This is a regression.

@dr2chase please make a cherry-pick CL once your change is merged in master.

[gopherbot]

Change https://golang.org/cl/132575 mentions this issue: [release-branch.go1.11] cmd/compile: in prove, fix fence-post implications for unsigned domain

[gopherbot]

Closed by merging ebf5d98 to release-branch.go1.11.