cmd/compile: improve regalloc live values debug printing

Before:

live values at end of each block
  b1: v3 v2 v7 avoid=0
  b2: v3 v13 avoid=81
  b3: v19[AX] v3 avoid=81
  b6: avoid=0
  b7: avoid=0
  b5: avoid=0
  b4: v3 v18 avoid=81

After:

live values at end of each block
  b1: v3 v2 v7
  b2: v3 v13 avoid=AX DI
  b3: v19[AX] v3 avoid=AX DI
  b6:
  b7:
  b5:
  b4: v3 v18 avoid=AX DI

Change-Id: Ibec5c76a16151832b8d49a21c640699fdc9a9d28
Reviewed-on: https://go-review.googlesource.com/109000


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>