cmd/compile: dense numbering for GP registers

cmd/compile: dense numbering for GP registers

For register maps, we need a dense numbering of registers that may
contain pointers of interest to the garbage collector. Add this to
Register and compute it from the GP register set.

For #24543.

Change-Id: If6f0521effca5eca4d17895468b1fc52d67e0f32
Reviewed-on: https://go-review.googlesource.com/109351
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>