Fold the comparison when the SHR result is small.

Useful for:
- murmur mix like hashing where higher bits are desirable, i.e. hash = uint32(i * C) >> 18
- integer log2 via DeBruijn sequence: http://graphics.stanford.edu/~seander/bithacks.html#IntegerLogDeBruijn

Change-Id: If70ae18cb86f4cc83ab6213f88ced03cc4986156
Reviewed-on: https://go-review.googlesource.com/21514
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>