cmd/compile: eliminate more allocs in newblock

cmd/compile: eliminate more allocs in newblock

name       old allocs/op    new allocs/op    delta
Template         389k ± 0%        386k ± 0%  -0.84%        (p=0.000 n=10+10)
Unicode          323k ± 0%        323k ± 0%  -0.25%        (p=0.000 n=10+10)
GoTypes         1.17M ± 0%       1.16M ± 0%  -0.93%        (p=0.000 n=10+10)
Compiler        4.13M ± 0%       4.09M ± 0%  -1.05%        (p=0.000 n=10+10)

Change-Id: I6c00850d07511c2e65761c7373fc3df738499105
Reviewed-on: https://go-review.googlesource.com/32235
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>