Previously, signed and unsigned integers could not be compared, but
this has problems with things like comparing 'x' with a byte in a string.
Since signed and unsigned integers have a well-defined ordering,
even though their types are different, and since we already allow
comparison regardless of the size of the integers, why not allow it
regardless of the sign?

Integers only, a fine place to draw the line.

Fixes #7489.

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews
https://golang.org/cl/149780043