Teach SSA about the cmd/internal/obj/$ARCH register numbering.
It can then return that numbering when requested.  Each architecture
now does not need to know anything about the internal SSA numbering
of registers.

Change-Id: I34472a2736227c15482e60994eebcdd2723fa52d
Reviewed-on: https://go-review.googlesource.com/29249Reviewed-by: David Chase <drchase@google.com>