cmd/compile: static composite literals are side-effect free

cmd/compile: static composite literals are side-effect free

This extends CL 22192.

This removes the remaining performance disparity
between non-SSA and SSA on the AppendInPlace benchmarks.

Going from non-SSA to SSA:

AppendInPlace/NoGrow/2Ptr-8  1.60µs ± 5%  1.53µs ± 5%  -4.04%  (p=0.000 n=15+14)
AppendInPlace/NoGrow/3Ptr-8  2.04µs ± 3%  1.96µs ± 2%  -3.90%  (p=0.000 n=13+14)
AppendInPlace/NoGrow/4Ptr-8  2.83µs ± 8%  2.62µs ± 4%  -7.39%  (p=0.000 n=13+15)

Previously these were 20% regressions.

Change-Id: Ie87810bffd598730658e07585f5e2ef979a12b8f
Reviewed-on: https://go-review.googlesource.com/22248
Reviewed-by: Keith Randall <khr@golang.org>