cmd/compile: optimize x > a && x < b to uint(x - a) < b - a when a, b are constants and b > a

[minux]

https://golang.org/cl/27032 demonstrated that by doing this
transformation, ~10% improvement can be seen in benchmarks
from the unicode package

[josharian]

I have a CL for this ready. Just haven't mailed it yet.

[hirochachacha]

That sounds good!

[gopherbot]

CL https://golang.org/cl/27652 mentions this issue.