[dev.ssa] cmd/compile: reducing alloc footprint of dominator calc

[dev.ssa] cmd/compile: reducing alloc footprint of dominator calc

Converted working slices of pointer into slices of pointer
index.  Half the size (on 64-bit machine) and no pointers
to trace if GC occurs while they're live.

TODO - could expose slice mapping ID->*Block; some dom
clients also construct these.

Minor optimization in regalloc that cuts allocation count.

Minor optimization in compile.go that cuts calls to Sprintf.

Change-Id: I28f0bfed422b7344af333dc52ea272441e28e463
Reviewed-on: https://go-review.googlesource.com/19104
Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Todd Neal <todd@tneal.org>