Previously comparisons with NaN led to contradictory results if it was
compared to anything not NaN, since Less always returned false, thus
breaking monotonicity of ordering.
This fix makes NaN less than anything else and adds NaN and (+-)Inf to
testcases.

Fixes #2092.

R=golang-dev, r, rsc, r
CC=golang-dev
https://golang.org/cl/4805051