cmd/compile: combine slice allocations in newblock

cmd/compile: combine slice allocations in newblock

name       old allocs/op    new allocs/op    delta
Template         394k ± 0%        391k ± 0%  -0.80%        (p=0.000 n=10+10)
Unicode          350k ± 0%        349k ± 0%  -0.27%        (p=0.000 n=10+10)
GoTypes         1.18M ± 0%       1.17M ± 0%  -0.92%        (p=0.000 n=10+10)
Compiler        4.18M ± 0%       4.14M ± 0%  -1.05%        (p=0.000 n=10+10)

Change-Id: I838a4e2110afe6496c535b9a0ec5aa882d63a707
Reviewed-on: https://go-review.googlesource.com/32223
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>