cmd/compile/internal/ssa: use math/bits for register sets

Using bits.TrailingZeroes instead of iterating over each bit is a small
but easy win for the common case of only one or two registers being set.

I copied in the implementation for use with pre-1.9 bootstraps.

Change-Id: Ieaa768554d7d5239a5617fbf34f1ee0b32ce1de5
Reviewed-on: https://go-review.googlesource.com/92395


Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>