This makes the cmd/compile/internal/ssa package
compile much faster, and has no impact
on the speed of the compiler.

The chunk size was selected empirically,
in that at chunk size 10, the object
file was smaller than at chunk size 5 or 20.

name  old time/op       new time/op       delta
SSA         7.33s ± 5%        5.64s ± 1%  -23.10%  (p=0.000 n=10+10)

name  old user-time/op  new user-time/op  delta
SSA         9.70s ± 1%        8.04s ± 2%  -17.17%  (p=0.000 n=9+10)

name  old obj-bytes     new obj-bytes     delta
SSA         9.82M ± 0%        8.28M ± 0%  -15.67%  (p=0.000 n=10+10)

Change-Id: Iab472905da3f0e82f3db2c93d06e2759abc9dd44
Reviewed-on: https://go-review.googlesource.com/41296Reviewed-by: Keith Randall <khr@golang.org>